I'm actually saying it is possible to sync your iPod from your iMac. I have not seen anyone report here that newer iMacs cannot sync to older iPods. If you want use automatic syncing, you will have to pick the computer you want to keep "in sync" with your iPod.
However, that does not mean both computer cannot have the same songs in their iTunes libraries. If one of them is considered your "master" library with ALL of your content, you can put that complete library on the other computer. This document explains how to transfer your iTunes library from one computer to another.
http://support.apple.com/kb/ht4527
See the method called External drive.
Another way (if you don't mind spending $25), is to use Apple's iTunes Match service. This puts a copy of your iTunes music library in iCloud, and makes it accessible from other computer and Apple iOS devices (such as iPod touch, iPad, and iPhone). From the iCloud copy, you play songs by streaming and also download any song (or all the songs) to another computer. I use iTunes Match, but keep a full copy of my iTunes music library on my primary Mac's drive. I can play my songs on other computers and devices that I own, from the copy in iCloud. NOTE: iPod's (except iPod touch) cannot sync with iCloud directly, so you'll need to have one computer running iTunes (with a local copy of your iTunes music library) for syncing with the iPod.
The subscription fee is annual, but downloaded song files remain playable even if you stop subscribing to iTunes Match. You just can't access the iCloud copy of your iTunes music library anymore, if you stop the subscription. (It also pays for ad-free use of the iTunes Radio service.)
I am trying to keep all my new purchases available on both computers and the Ipod.
You can download previous purchases from the iTunes Store at no cost, from another computer. Sign in to the iTunes Store using the Apple ID used for making the purchases. On the iTunes Store Home screen, click Purchased under QUICK LINKS. On the Purchased screen, you can find and download previous purchases. So even without iTunes Match or transferring your complete iTunes library, you can easily keep recent purchases "in sync" on both computer, and sync the iPod to one of those computers.
how do I properly clean off the iPod?
If the iPod is currently set up to automatically sync with one iTunes library, and you set up automatic syncing with a different iTunes library (on the other computer), that will replace the existing content of the iPod. So, there is no need to "clean it off." However, you can do a Restore, if you want to give the iPod a fresh start. In your case, if the iPod was previously used with a Windows PC, and you want to start using it with a Mac, a Restore reformats the iPod's storage for Mac use. It's not required, because Macs can use the Windows format. If you go back the other direction, it is required becaues Windows PCs cannot use the Mac format.
The Restore button is on the iPod's Summary screen in iTunes. It will erase the iPod, reinstall its software, and set it to default settings. It's what some people call the "factory" reset. You can then set it up to sync as desired.