Most larger Synodontis are territorial (rather than social) fish. The decorus are a notable exception, but few people really have a tank large enough to properly house a group of those, and the ones that do have such tanks are typically inclined to keep other, even larger species of fish in them. The FF are fine as singles, not so good with 1-2 other FF's, but generally good in a larger group. Groups have spawned in aquariums, but the eggs are quickly eaten and very seldom do any actually survive.
I have a single FF in my 125, a breeding group of multi's in a 75 with a group of peacocks and groups of petricola in a 55 with some SA cichlids and another 75 with peacocks and mbuna. So far I've raised 6 multi fry in the 75, recently sold the 5 larger/older young ones and still have a wee 1" fat baby in the tank. They grow much slower than the FF's.
Recently released multi fry:
Sad looking little fish, all head and hardly any body yet...
Grown out for about 6-7 weeks:
It doesn't show in the pics, but fry/juvies that have been out of the female cichlid's mouth for a day or two have relatively huge, round bellies and waddle when they swim. They find every available scrap of food and eat constantly. Even before the tank is fed (once daily) they're already stuffed, and still eager for more.