I got my new iMac about a week ago. I ordered it four weeks before it shipped. Let me try to answer your basic questions.
Your Finder desktop has two hard disk icons. On my system, one was named Macintosh HD, and it was the SSD. The other was named Macintosh HD 2. I renamed it to iMacHD. All files and the OS are on the SSD. The conventional HD was empty.
When you boot for the first time, you get to Setup Assistant. I quickly realized I could not migrate my old system to the 256 GB SSD. It was much too large. I quit the setup and began studying how best to proceed. I Googled some tips and watched Don McAllister's screen casts about setting up a new MacBook Air with SSD. It all wasn't directly applicable to my iMac, but I got the idea. You have to figure out what you want to store where.
I finally settled with having only the OS and my apps on the SSD. All the rest of my stuff I copied to my iMac 2TB HD. I moved my Aperture library (iPhoto works the same) and did the same with iTunes. They both operate off the HD and not the SSD.
After all this, I found some of my apps didn't recognize they were already installed. That was because I couldn't migrate my Library folder because it was too big. Make sure you have you license info handy.
That's about it. The speed is amazing. Apps load almost instantly. The SSD is great.
Your SSD machine will be build to order, so you have some time to organize things upfront. I ended up with a 'system' drive containing all my essentials that was ~180GB in size. When the new machine arrived I ran setup assistant with a Carbon Copy clone of my system drive and after 2 hours (the CCC clone was on a external USB drive) everything was fine.
Maybe this will help, although it's Aperture, not video editing.
I have made a "second" home folder on the HDD. This has the same read/write restrictions as the real home folder, and has links in the Finder Sidebar.
I put the Aperture Library in the SSD home folder, and most, but not all, master images in the HDD home folder. Recently added images are placed in the SSD-resident Aperture Library. As the library grows, I will "demote" masters to HDD to make more room. By that stage, I should have finished most classification, rating and editing of those images. I may keep recent "selects" (say 5 star) masters on the SSD.
Thus the library metadata and other small files are on SSD and the big files (RAW images about 25MB each) are either on SSD or HDD.
Large files don't get as much of a boost as small files when on SSD. I imagine that large video files will get some benefit from SSD, but I've seen my HDD writing data at up to 150MB per second so it's not bad.
So I get SSD speed for the following, in increasing order of importance
Initial app loading
Subsequent app module loading
Paging in and paging out
Library constructs, database, metadata, previews, thumbnails, adjustments
You may be able to translate these notes into some relevance for your video work.
Hope this helps
"Documents" is a folder, not a file. It contains files.
You can delete the contents of those folders that Mac OS X wants to keep. You don't need or want to delete "Documents" or any of those folders directly under your home folder.
I store small files in the Documents folder on the SSD, things like typical Word or Pages documents, or typical spreadsheets. It takes a lot of them to eat up a significant amount of space.
Larger files, like music, large image files and videos are stored on the HDD where I have a folder structure like this:-
--------Virtual Machines (for VMware Fusion)
Those subfolders of "John" have their own subfolders of course, with a structure of my choice.