-tetras need to be kept in schools of at least 6
-Kuhli loaches like to be in small groups also "recomend at least 6"
-maybe hold on the platys and get more tetras, you will have a more impressive display with a single species. if you do go with platies, i recomend at least 6 to disperse any agression among the group.
-Gourami depend on the species. For most species, you can have 1 per aquarium. Some species out their, you could add more but in a 29 gallon, it would be cramped.
You have enough space for all of those. Here's an ideal stock:
~ 6 bloodfin tetras
~ 2 - 3 sparkling gouramis (dwarf gouramis are okay too but are overbred so can easily get diseases)
~ 5 platies
~ 3 kuhli loaches (trust DutchAquarium more - I don't know much about loaches. Six, however, is a lot and is not needed for them to be happy. 4 or 5 would be ideal)