The reason why i ask is because i bought 1 at 12'' a few years ago that was raised in freshwater,i kept him with a 12'' Channel cat and it worked out fine.
never heard of a salt water frontosa. Ive kept madtom catfish, plecos, channel cats( cats got to be much smaller than fronts or they will out grow and eat fronts)
synos are probably about the best mostly because they are naturally found together (or close), but I keep mine with polys and the others listed in my sig