it's 25% chance for 4/5 to 5/5 using rainbow, so yeah 21 fails is just an example of how unlucky one can get.
as for lauren...well eventually you'll get enough of him. generally there isn't any rush to SU any unit, so you don't need to rush to use rainbow fairies. just expand your slots if you need space.
also, sherry is good, probably even better than thetis. once her AW comes, sherry buffs all gold and below units I think. thetis only gets status immunity (immune to poison and paralysis) which is good primarily for bosses that will only be introduced at a later stage.
but since they're copies, I guess you can just use them as CR fodder for their mains.