I got a baby rtc in a 40 gallon, I put him in there with 2 4inch phantom knife fish amd 2 x-ray tetra's. He is 9inch now and his 120 gallon is almost at the end of its cycle and i will soon convert. I even had to "starve" him to balance the nitrite levels and everyone survived with him hungry. They share the cave and eat bloodworms head to head. Though i do really like my knife fish so i wont put them in the 120 gal. Way to much risk.
Point being size, age, temperment all play factors in wether the RTC will eat the other fish. Also im pretty sure the P-bass can EASILY out run the RTC. Mine sprints when i transfered him and hes pretty slow compared to most fish.