I'm about to buy a new HD for my good old 1200, and this time I wanna play around with OS installations... 
Basicly I just wanna have 2 partitions, one for the OS stuff and one for everything else. Is there any way to install multiple systems in a single partition? I mean: 3.1, 3.5/3.9
No reason you can't do this... I did it before. What you'll need to do, though, is hand install things... And I made a set of "virtual" roots, and assigned sys: and the others to whichever one I was booting from. in other words, on dh0, I had...
dh0:s/
dh0:c/
(This set contained the startup and selection code. I used a really cool AmigaOS looking startup manager, too. I can find you the name, if you like. it was on Aminet, somewhere.)
anyhow, for each OS, I had a directory with the subdirs under it, like...
dh0:os31/ (This would be assigned as SYS: if I was booting 3.1)
dh0:os31/s (3.1's S: directory)
dh0:os31/c (etc...)
likewise,
dh0:os39/
dh0:os39/s (etc...)
Certainly, it can be done. It was a bit of hassle, but a unique system. You also have to be careful when installing programs, cuz some installers will grab for dh0:s/startup-sequence, instead of your assigned startup-sequence. And therefore, modify the wrong file....
and even 1.2/1.3 (provided I've got a Devs/Kickstarts/ full of grabbed roms 
That's not gonna work, at all. As soon as you kick an A1200 to 1.3, you lose your scsi.device (AT hard-drive support). Hence, no hard drive available, at all, from that point on... ;-)
