Well, the dart frog community will likely chew me out for saying this, but if you insist of multiple types of darts, how about tincs of various colors and markings. At least this way they will be all the same species and you will have variety. There are blues (azureus), Oyapoks, citronellas, powder blues, cobalts, reginas, and so on.

Why the dart frog community frowns upon this is because they do not want any "mutts" mudding up the gene pool. They, like myself are purists. We don't like hybrids and the like. We like things the way nature created them.

Now if I were setting up that tank I would either house blues or Oyapoks in it.