They require very different water conditions so one will do better thatn the other. Strictly speaking you shouldn't mix africans and new world cichlids.
7.5 ph is within the acceptable range for both fish, maybe it's one of the exceptions when it comes to mixing old/new world cichlids. I think a decent number of fish might help control aggression when they pair off and breed, and I agree with Minnnt about the convicts - they can go totally psycho when they have eggs. Your tank sounds great, I imagine the yellow lab and convict mix looks striking - good luck I hope the tank does well.
The labs are probably at the top because they're scared to death of the Cons They're seeing a strange looking fish and are unsure of it. I would go with one type of fish or the other.
Both fish will survive at 7.5 but neither will thrive imo. If you want Labs then go with a coral/limestone substrate to buffer your ph a bit, if you want convicts then add some normal substrate and add bogwood in an attempt to lower the ph.