New Fish

Great that you have decent-sized shoals of harlequins and cardinals.

Looking at the species profile for dwarf rainbows, they should indeed be in a shoal of at least 6-8, they need a tank of at least 60x30cm footprint, and the fish grow to a size of 8cm "although usually smaller in aquaria".  You possibly could just about manage a shoal of them. However, with the ottos also in the tank, I'd say that getting a shoal of dwarf rainbows is a bit risky stocking-wise, especially as they grow quite large - and are quite active. I've seen them in the LFS and certainly wouldn't consider even the juvenile-sized ones in my tank of that footprint given how actively they swim, lovely fish though they are, never mind adult-sized ones. 

I'd advise you to return the lone fish, and, if you were to get any more fish for your tank, then maybe some additional ottos in time once you're confident the tank is mature and has sufficient biofilm/algae for them to graze on.

Others who have had these fish might be able to advise further.

I'm also in agreement with this.  Perhaps the store might bw generous and you could swap the fish for another otto for your inconvenience?