Good idea about checking with the store if you can return/exchange if the fish doesn't work out. It's always good to have a plan, whether it be moving to another suitable tank or returning to the store.
I believe rainbows are pretty active, so I would think that the larger ones would need more room. The smaller ones like the Praecox may be an option.
Oh, and I've never seen my honey gouramis even nibble on any of my plants (anubias, java fern and moss, ludwigia repens, bacopa monneiri, duckweed, frogbit, pennywort, variety of crypts, etc.) I'm not sure if the others would be more likely to eat herbage, and I know they are omnivorous. Perhaps they are just getting enough vegetable matter from the food mixes I feed.