Too Large Fish for Tank
I recently completed a fishless cycle on a 20 gallon high tank, my very first venture into fishkeeping. I went to my local fish store today to finally purchase my first couple fish. Honestly I didn't know much about what species would fit well with my tank, and I didn't do much research because I expected that a lot of fish that I would read about online would not necessarily be available at this (rather small) store. I also expected the folks there to be able to recommend pretty well what would fit well in my tank. So trusting this, I bought two Lake Kutubu Rainbowfish (Melanotaenia lacustris) on the recommendation of the guy at the store. I tried to do a quick google on my phone in the store to get some background on the fish, but of course there was no cell service in the store.
Only upon getting home, drip acclimating, and introducing them to my tank do I start to do research into my new fish and discover that they typically reach 4.5-5.5 inches and have a generally recommended tank size of at least 55 gallons.
Clearly my tank is not a great match for these fish, but I'm very new to this hobby, so what is the protocol in this situation? Will they be healthy and comfortable enough in my 20 gallon, or should I try to return them to the store tomorrow? My instinct is to return them, but I'm not sure if fish stores even typically do that kind of thing. I'm also hesitant to put the fish through the stress of another move back to the store if it's not necessary. Any advice would be much appreciated.
For clarification, at the moment they must be quite young because they're slender and a little less than 2 inches long. If only they stayed this size they seem like they would have been a great fish for my tank.