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15-20 is a bit ambitious! I know they are small, but in a 6 gallon, they wouldn't have any room to swim :/ I'm no expert, but I have kept them before. I had two in a 3 gallon and they were okay So maybe go with 5 or 6? Not sure though, you'd have to wait for someone with a bit more knowledge
Not sure about the oto/cory, but a Nerite Snail would be amazing for algae eating if thats what you want the fish for? I have two and they're amazing!
Oh and obviously because they're livebearers, they will have babies like crazy if you keep males and females. If you dont want babies get only males because females can store sperm so could have babiies even after you've bought them.
If you want babies then you'll want to get about 2 or 3 females for every male. So probably 2 males and 4-6 females. You'll need somewhere to get rid of the babes to or you'll be quickly overstocked. You could ask at your LFS if they'd accept them for store credit/other fish/money. Or just keep them and get a bigger tank!
Like I said, I'd stick with 5 or 6. To me that seems like a pretty safe number. You can always add a few more if it looks really empty and the parameters are okay - as in 0 ammonia, 0 nitrites and 5-40 nitrates.
That reminds me, is it cycled? And have you been adding a source of ammonia to stop the BB from dying off?
Yes it is cycled but had an ammonia spike which I have lowed to 0.25 but I am thinkin of re homing my 5 guppies and getting endlers as I think they will suit this tank size better?
Yeah, I agree that you should get rid of the guppies. By that I hope you don't mean killing them? Take them to your LFS. Endler's would be much better as they're basically the same thing but smaller.
Also, keep doing daily water changes until the ammonia is at 0 constantly, because even a small amount will harm your fish. THEN, get your Endler's.
I wouldn't say max. but get 6 to start with. f your water parameters are okay after a few weeks, buy 1 or 2 more. But then I would say that is your limit.