i only know much about agassizi, kribs, and rams because they're most common, but i'm pretty sure most apistogrammas are similar. They do best in harems with one male to several females and plenty of hiding spots, in 30-40 breeders but in a tank your size you may get away with around 3-4 males if you're careful. they are generally peaceful but males are very territorial with one another and other similar species. One male for 3-5 females might be a good ratio. they're pretty shy feeders so a smaller tetra population then you invisioned would be good if you can assure the cichlids are eating. If you want to focus on the apistos get smaller and/or more timid tetras and avoid the very boisterous ones like sepraes, buenos ares etc... I suggested bleeding hearts because they are pretty big and laterally compressed with nice colour while also being very timid, a good mix for apistos. In your tank I would start with a school of 20 timid tetras and see how the apistos fare. If they get plenty of food I suppose you might be able to bump up to 40-50 tetras assuming you have lots of apistos and cories and heavy filtration. the top tetra candidates for your tank IMO would be bleeding hearts, black phantom, pencilfish, cardinals, neons, rummy nose, emperor tetras, and hatchets because of their size and/or temperment. I'd pick your favourite 2 species and get get a school 20-25 strong of each once the cichlids have adapted to your tank. a neat setup but the apistos are sensative to dirty water and you will need alot of filtration for this sort of plan.