If the rats from the different litters don't seem to get along right now, that's ok. Just leave them in separate cages, and let them play out together (rats need exercise, and it's a good thing for them to have a safe room that they can roam around) in neutral territory. Neutral erritory can be your tub- they've probablu never been there, and it's small enough that it a fight happens, you can separate easily.
Eventually you can switch cages for a few hours, because the rats get used to the strange smell. More play, and at one point what I ususlly do is I leave both cages open. At that point, I supervise like a hawk, since fights can still happen in the cages, the live-in rat(s) may not tolerats the other ones so well. If that happens, wait some more, more play together.
Of course if the rats just never get along at playtime, well... Just like pit bulls and your degus they may have to live separated.
I feed my rats lab blocks -Harlan Teklad is a good brand, or any of the lab blocks that are NOT corn based... Also Suebee's mix (see the ratsrule.com link) and a Red River cereal along witl oatmeal. Add tons of fruits and veggies, too many treats...