Any brichardi will most likely spawn a lot for you, you just need to take some out once and a while or else the population will explode. They are really cool and take care of their young so you dont have to do that much.
On the other hand, i have a 40g tank with 6 full grown calvus, 3 of them are lfs fish and the other 3 are wc. The wc are 1 male and 2 females, each female breeds atleast once a month. They do not really take care of the fry once they become free swimming, so if i want them to survive, i put them in a 10g.
You could also have a shell dwelling colony that would most likely breed for you. Or a colony of cyps.
The possibilities are endless, you just need to put in the time for them to mature and be happy.
hope that helps
-bboys