I think you are plenty fine to have 6 apistos in a 29 gallon - what is that, 3 feet long. You will get a deal of displaying between the males, and you should have space for the females to setup microterritories of 4 , 6 inches diameter. I thnik the most I've had in a 4 foot tank was about 30, to try to recreate how they live in the wild. Very interesting setup.
When you order a bag of mixed apistos from a wholesaler , what would you get? I doubt you would literally get an order of truly mixed apistos. I suspect you just get what they have lying around, or have not, cannot be bothered to species id. My lfs frequently gets bags of mixed, unknown apistos from S America - they are normally all much the same, just unid'ed. Be aware that any bag of wildcaught apistos (or cories) will always have a certain percentage (5 -10) of 'others' mixed in anyway , which may or may not be more interesting.
Just go to the lfs and see what came in a particular bag on a particular day.