I had my O beta with a waspfish for a while. The waspfish was about 2/3 to 3/4 the length of the toadfish, and they never seemed to have a problem. So I do think it would probably be a safe bet as far as not trying to eat the Inimicus. However, toadfish are very aggressive feeders, and may outcompete the devilfish for food, stressing them that way.
If you have the room for another tank, I think a single tank for the toadfish would be best.