I kept 6 redbellies and 1 rhom in a 150 gal. The rhom kept all 6 reds in one corner of the tank and tore their fins up.The reds being in the corner all the time would then fight with each other.The reds were about 6 inches the rhom 5 inches.I had to put the rhom in a 55 gal or the reds eventuaoly woul be killed.They were no match.
thats some pretty expensive experiment there. thats the only way to go is to have 2 tanks if you want to keep your serra and pygos. even if you had enough money to get the largest tank you can the serra will dominate the tank. serras do not like any other fish in their territory and eventually the whole tank will be the serras tank. for the fishes sake and happiness get another tank. i hate to see serras or pygos get messed up for an experiment.