The only true top level Tanganyikans are Cyprichromis, but you have them in another tank, and 3ft isn't long enough for them so that really leaves Rock Dwellers or Shell Dwellers. Shell Dwellers are going to be on the floor so that's ruled them out.
Probably one of the Julidochromis family would be best if you have a good rock/cave area for them. You could keep a pair of J.transcriptus, J.dickfeldi or J.ornatus inthere with the Synos. J. marlieri and regani get too big for a 3ft.
Another option would be a pair of Telmatochromis vittatus or you could go for Neolamprologus Caudopunctatus - you will need to provide shells for spawning if you go for Caudopunctatus though. They don't live in shells, but do prefer to spawn in them.
A 3 ft tank makes it hard. Julies are predominantley down low and will be agressive if they breed.Unlikely to be a problem for your synos. Have you considered the calvus? Slow growing and less likely to breed but a fantastic fish all the same.