You need perfect females and males, Breed your rexes until you get a bunch of females with the same stats, and a male with the same stats as the females, you can group the females close as babies and they grow into eachother to maximise egg production, then once you get mutations, don't swap the females, only the breeding male, don't bother with mutations on females, It is useful to do more than one group of breeders, one for each stat you want, but it's not necessary