Like Kyle said, it is not a good idea for a beginner to mix species. It seems that this is what goes through a mahority of our minds just before we actually acquire frogs and I an truly understand your desire to do so. However, after you have gained some good insight and experience with darts you may be able to mix a couple of species of seperate genus such as one species of Dendrobates and one species of Phyllobates. Also, though I'm sure many will argue against this, I believe that one should start with established, healthy juveniles first. If you are successful for a good while, you can consider adults. The reason I say this is because when and if you do breed frogs, you need to be prepared with hands on experience in caring for such small frogs. Best of luck with your upcoming project(s) and welcome to this fine hobby!! 
-Bill J
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Updated list as of: 12/13/03
2.2 D. azureus
1.2.7 D. ventrimaculatus
3.3.1 D. tinctorius 'Suriname cobalt'
0.0.3 D. tinctorius 'patricia'
0.0.1 D. tinctorius 'giant orange'
0.0.1 D. tinctorius 'citronella'
0.0.2 D. auratus 'Panamanian'
0.0.5 D. auratus 'green/black'
0.0.3 D. imitator 'Alex Sens line' (very soon)
0.0.2 D. reticulatus (soon)