Out of 9 fish total ( 4 being juvies) what sex ratio's do you have? over the next 12 months you could probably establish
2 breeding groups (so maybe another 3ft tank is in order)
9 fish in you 3.5ftr should be OK for a while but since your tank is low profile( 15inch) you have a smaller water capacity
compared to a tank that's 20inch tall(obviously), so with 9 B/N in there you're going to be doing a fair bit of maintenance
and like deano said "I hope your filter is up to the job"