Great decision, IMHO.
I used a TBS package to set up my first tank. A 20g. Tons of critters. No mantis shrimp. But plenty of whelks, keyhole limpets, porceline crabs, pistol shrimp, urchins, sponges, tunicates, barnacles, etc. etc.
Some I removed as quickly as I could — the whelks (so they wouldn't prey on my CUC snails) followed by the keyholes (they began to munch on my acans).
Some I removed later on — a rock urchin (began to get too big for the tank), a polyclad flatworm, and some gorilla crabs (Then I got lazy and just let a few other stay. They never bothered anything.)
And many I was glad to have, and kept — pincushion urchin, the little pistol shrimps, all the rest.
Yes, some of the hitchhikers resulted in the need to alter or delay some of my other stocking plans for the tank. Or caused an occasional noobie freak-out ... until I did some reach and found out my latest discovery was nothing to worry about. (Nothing ever truly was.)
But it was, by far, the most interesting, engaging, diverse, and stable tank I've had.
Every evening, for months on end, my wife and I gaze into it and discover some new and curious critter to marvel at.