Can two different species of fish breed?

No they can't. Fish breed differently than mammals do and they lay eggs which are then fertilized by the male fish. Your fish getting big was not it getting pregnant because fish cannot get pregnant. The fish had just produced a bunch of eggs and was carrying them around until it found a suitable place for them to hatch. It probably laid them in the rocks and you didn't see them but they didn't make any other fish because there was not a male of the same species in the tank.