Either all the rats I've ever had have been weird, or the 'meat makes them aggressive' thing is a myth. Our rats are very friendly, and we give our pet rats meat (we have about 20-30 pet rats which are not used as breeders, they're just pets, yes, I know, it's lame :lol: ). The rats being used for snake food are on a more strict and consistent diet, but our pets get just about everything you can imagine, and we haven't found anything yet which will hurt them. If you stick a whole roast chicken in there they get so excited and devour the lot - bones and all. The other day we put the remains of a roast duck in with them, and they were thrilled! They never get the slightest bit aggressive towards each other or us.
Rats are naturally semi predatory, and if they don't get enough protein they'll breed poorly and often eat their own babies. It's not natural or healthy for a rat to be vegetarian. Any good rodent pellet will have some animal protein in it.