Hi,
There are now some very interesting discussions going on this thread. Let me see if I can help with some of your questions.
1) you won't have any issues with the synology as it is a NAS device it doesn't have to be formatted in the same way as a mac. You setup a folder structure and create network shares that can be connected to from pcs ans macs alike.
2) what to serve with this is probably the biggest question with several answers depending on your preference. I have a bit of a combination going on. On the synology there are a number of ways you can share media using DNLA / upnp
So devices like Xbox or PS3 can access directly. The problem is the user interface isn't slick, jus like browsing folders.
You obviously have iTunes, great for music and multi room setups with the Apple remote app for instance. But limited when it come to movies and tv and no live tv pvr if you want that. plex is great and has a lot of supporters. I'm not an expert but i do know Plex has 2 main parts the backend server which manages the media and then the front end which is the slick user presentation. You can have these parts together on the same machine but you can now run the plex server directly on the synology which is a great idea for centrally managing your media.
I have only dabbled with plex at the moment and am sterling away as I want my solution to include live tv and pvr. This is not the main use for plex, there are some add ons which I have not tried but understand they are not as slick as some others. I will mention some more about tv later.
3) where to manage the media, this for me is where the synology comes in, I personally don't like iTunes 'managing' and 'organising' everything for me. I like to know what I have, where it is, how it's tagged and importantly want the choice to move away from iTunes at a later stage if I want to or share my media through other means at the same time.
So what I do is have a number of media shares on the synology music, movies, home videos, pictures, tv. I can shares there via upnp, DNLA. I also point a iTunes instance running on a mac mini at the same shares but I don't pull the data into the iTunes library. This gives you the full iTunes capability of streaming to other apple devices, syncing iPads etc. I have also moved the iTunes library (the actual database, album art, playlists etc) to another shared on the synology called iTunes funny enough. There are forums on how to do this but if anyone is interested I can post something.
So this way the mac mini is just a presentation device with no data on at all. So you can replace without having to find and move your data. Also another good point on this is that you can have more than one iTunes server using the same media and iTunes library/database. So instantly you get the exact same iTunes library, playlists, album art etc. on more than one iTunes server. Any updates you do appear on the others. There are some gotchas with this though. Firstly all the itunes servers need to see the same media shares. You can't edit manage the iTunes on more than one machine at a time you have to close the others while you make changes on one, once you make the changes you can then reopen the others. I have also experienced issues with doing this method between macs and pcs, I think because the sharing path for the media between macs and pcs look slightly different so when I try to use on a pc it looses the media, but I will look at this at some point.
4) home videos etc, this is where other products such as plex, media portal and xbmc come into there own. I have been playing around with a few things and am now working on XBMC, probably one of the first media centres back in the day when you could hack the black Xbox stick in a massive 120GB disk and install xbmc. Probably where Microsoft got there media centre from. Anyway I digress, xbmc does everything, music, videos, tv etc. what I particularly like is the ability to integrate live tv pvr functions into a single media centre front end. I want to ditch my bt vision box, and have everything accessible from a single interface that my mum could use.
I am currently focusing on the tv side and have been playing with a number of options. I do have a win7 server with dual tunes running media centre and media centre extender on the xboxe and want to re create this and switch that big beast off. I've played with The eyetv tuners on the mac min, ok but didn't want multiple interfaces, you can get a plax add on for eyetv but I understands it isn't the smoothest. Then I stumbled across a couple of pvr solutions that can be run on the synology as backend servers and then connected to via multiple front ends such as iPad apps, xbmc for instance. Thes are DVBLink and tvheadend, both of which can be installed on the synology and you use a USB tv tuner plugged into the synology also do there own 'video station' but ther are no media centre plugins yet.
I have tried DVBLink and tvheadend, I could not find any tv channels in tvheadend but have had success with DVBLink and also with the xbmc plugin. So I have now ordered 2 x DVBLINK-T2 tuners so I can get free view HD and plan to build this over the weekend using DVBLink and xbmc.
Now here is the really intesting part (for me anyway) xbmc runs on several platforms, not just macs and pcs. There seems to be a really version version that people have running on the rasberry pi. For those of you that don't know it is a very cheap £20 micro computer with HDMI output and smaller than the ATV. designed For education and hobbiests there is some good stuff coming out on these. I bought one a while back to have a play with and may just have found the perfect use for it.
So if this works, then I am thinking the mac mini will not need to be on much at all, and I may just use for a central music store and around the house streaming, which seems a waste for a 16GB i7 with 2 x 1TB hdd but lets see how the xbmc And the pi work out.
5) photos, again down to preference, once you have a picture share on the synology you can use what you like, plex, xbmc, synology has something called photo station. You can use one or many tools to share and present to same source.
Just a couple More points around synology if you haven't got one yet consider what you want to use it for, if you want to use it for running service on such as tv pvr etc, then pick the more powerful options, I have the ds212+ which has better processor and memory. I don't use raid anymore, I used to but decided that I still needed to have backup somewhere else and there was a big cost difference in running 4 bay synology with raid 5 (you need to use the right disks, don't use Green disks for raid so disks will be more expensive) opposed to running a 2 bay with no raid. 1 have 2 x 4TB disk so have a 8TB NAS. As I fill it up I will get a second as a backup and keep it in a different part of the house and sync them.
You can also use synology as a time machine, plus there are countless other services you can run on them such as CCTV, mail server, BitTorrent server. They are a real powerhouse.
Anyway hope you have found this useful. Happy to post more on my own progress as I go and if anyone wants anymore information on the above I will try to help.