I'm not exactly sure bichir's are compatible (in the long run) with zebras, but let me get to your question. Zebras do better in all-mbuna tanks (or at least their tank mates do).
Eggspots on zebras aren't indicative of gender. I have a female red zebra with at least eight eggspots (she's produced fry before so no question as to her gender).
Red zebra males tend to develop a lighter coloration (more pink-orange than 'carrot orange'). Additionally their bodies may have a blue sheen, and their fins, particularly the dorsal, have distinctive blue hues.
Cobalts are a little harder to determine (at least for me).
Venting is determining gender by the physically inspecting the reproductive openings/area of the fish.
With both species (reds and cobalts) though, males typically have pointed dorsal and anal fin tips (as opposed to the more rounded find ends of the females). Keep in mind that sub-adult males may 'appear' to be females (which means gender determination among juvies is even a harder task).
Some may say the risk of creating hybrid zebras is considerably increased when you keep several zebra species together, though I have yet to experience this in my tank (I have reds, a cobalt and some BB 'chilumba's [see avatar] in the same tank and the species ignore each other), perhaps due the the sex ratio I maintain them in.
Are the silver dollars intended for this tank or a separate tank?
