You could have danios but the gouramies probably won't appreciate the intrusion. Guppies I would avoid as they are liable to get nipped.
An alternative to guppies would be a trio of platies (i.e. one male, two females) or even a larger group - just make sure there are at least 2 females per male. If you stick to a small group you could also fit in some schooling fish other than danios (because danios are top-dwelling just like gouramies prefer to be). I would recommend something on the larger side to disuade hungry loaches who will snack on them given the chance - though this is uncommon. Black phantom tetras are one such option and IMO gorgeous (males have beautiful fins and change colour from dark to pale very rapidly and do some pretty displaying to settle hierarchy issues), chequered barbs are another, pristella/x-ray tetras also make a lovely shoaling option. If you want more fish but not necessarily shoaling species, cherry barbs do well as trios or pairs. American-flag fish are also quite charming - be careful to obtain a male/female pair though cause they can get very aggressive towards their own species.
Obviously, don't get too many fish or you'll be overstocked. I would suggest just two species of which only one should be shoaling, if any, and stick to the lower limits for group size (e.g. 3 platies and 5 black phantom tetras).
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