Those are a poor first time dart frog choice. They're pretty shy, very small, and if you've never kept dart frogs before, the small ones are very delicate and easy to kill. I suggest you look at something like Dendrobates tinctorius (including azureus), D. leucomelas, D. auratus, Phyllobates terribilis, or P. bicolor. They are larger, much tougher, and quite bold - you will see them, while you wouldn't see Ranitomeya ventrimaculata. The general thinking on mixing species is don't. Some dart frog species are quite territorial (D. tinctorius is perhaps the most territorial) and most species can stress each other, even if it they're not very territorial. There are also possibilities of disease transmission, parasites, and mutual toxicity.