30 rats are plenty if they selected them properly - different gene pools, different breeders etc...

You wouldn't necessarily care about genetic diversity for this sort of experiment. This experiment isn't trying to say "all animals" or "all rat species" exhibit this behavior; the interesting thing is that any animal does.

For me, the more important question is not whether all test subjects were genetically diverse: it's more important that the free rat and the caged rat are dissimilar. The situation would be vastly different if the rat pairs were related, since there is a genetically selfish motivation for caring for those within the same gene pool.

This becomes apparent when you try to co-house rats and mice. Litter mates are much less likely to fight.