I don't think it's a good idea to mix these species. A few problems:

1) Bullfrogs and toads have different environmental needs; a tank that is well set up for one is unsuitable for the other.

2) Bullfrogs get big, fast; they may try to eat your toads. Even before they're big enough to choke down the whole toad, they may try to eat toes.

3) Toads produce potent skin toxins; if your bullfrogs harass the toads, or even just rub against them, they could become quite sick or even die.

Toads can be good pets, but if you want to keep them, I suggest you give them a tank to themselves.