[depends what size bichir you are getting, small ones i have found get on with any fish if you keep them with them from a young age, your speaking to someone who has...
In the wild, Synodontis cats are a normal part of a bichir's diet so, if the sizes are right, the bichir will dine.
That being said, I keep various species of Synodontis in most of my bichir tanks without incident, though I'm careful to tailor the size of the cats to the size of the bichirs. S. petricola should be OK with P. senegalus unless there is a great size difference.
My juvenile albino birchir chases my 1 1/2" cories a little theres no way on gods earth he can fit them in his mouth (he cant even eat those sinking wafers for cories whole) and the cories will go into another tank in 3 months (ie before the birchir grows to eat them)