Its okay to mix different types of frogs to a extent.
For example I mix White's tree frog with the Dumpy tree frog since they are pretty much the same just different colors.
Another example is you can mix red eye tree frog with the clown tree frog or a small green tree frog. as long as niether of them were caught from the wild so they can pass disease to eachother.
Just make sure any frogs you are mixing come from the same family and not bigger than the other.
A poison dart frog should only stay with its own kind.
What types of frogs were u thinking about getting?