Help transferring files to used iMac from older iMac
I have a new used iMac (21.5 inch, late 2009), without gray install disks, that I just erased and installed OS X 10.6.8 from install DVD (with guidance from Apple Care tech). I want to transfer everything from my even-older iMac (20 inch, Mid 2007) using Setup Assistant, however I now realize my even-older iMac is running OS X 10.8.5. Can I transfer this way and then update the newer iMac's OS? Or, is there a way to update the newer iMac's OS and run Setup Assistant again? I've read so many posts about problems using Migration Assistant that I'm afraid to go that route instead. I appreciate any help!
iMac (21.5-inch Late 2009), Mac OS X (10.6.8)