Eight frogs of one species would be best. The two species you mention are of two seperate families and locations. Most dart frogs these days are captive bred and are usually parasite free. Mantellas, however, are still wild caught and are likely to be carrying gastrointestinal parasites, so you risk infesting your darts with an exotic parasite they would not normally encounter.
As most people know, most captive bred dart frogs lack the toxins of their wild counterparts. Wild caught mantellas are toxic and their presesnce in the same terrarium will most likely poison the darts. If you end up getting wild caught auratus, then they can poison the mantellas too. This is the number one reason why we do not house different amphibian species together. You would run the risk of losing both populations.
Lastly, where are you finding golden mantellas for sale? I have not seen one offered in years, which is too bad cause I wouldn't mind having them again.