The 600 will definitely need more RAM. Workbench will use part of the default 1MB, leaving you with very little - probably not enough for even the lightest of the lightweight WHDLoad installs.
You can do a bit more with 2MB, but it's still tight - you'll need to turn off the WHDLoad preload option in most cases, which will be a big performance hit for disk access speed.
I've got a total of 6MB in my 600 and it's enough for most WHDLoad stuff.
is it possible to boot the 1200 as a 500+ or 600 with the rom version 2? i can only find 1.3 kick
Yeah - look for SKick on
Aminet. There's no reason to, though. I'm not aware of any software that runs on 2.x but not 3.x. Plus, you'll lose access to your hard drive - the driver isn't in the 2.x ROM (I think).