Figure eight puffers are brackish water and senegals may not agree with that. I've kept fig 8s with other species but they have a tendency to sample other things, that being they may randomly bite other fish out of curiousity. Sometimes they take a dislike to something and then it gets more serious.
If you wanted to try to keep something with fig 8s i'd go with something brackish water and reasonably fast or hardy. I've used mollies, monos (while they were transitioning to marine conditions) and scat (likewise while moving to marine).
Over all puffers just do much better by themselves, they eat a lot produce a lot of waste and are rather unpredictable in temperment. If the tank is large enough you might be able to pull off a couple figure eights together but i wouldnt crowd them and i'd watch for aggression.
jason