Tiny One-Shot, players expressed interest in Gremlins and Redcaps...

Well, the Redcap would be modded to be tiny rather than small, as would anything else that is not tiny, but it was originally conceived as a break from the regular game still within the same storyline. Them playing the gremlins and such would be a witness's testimony as to the event the regular party would have to deal with.

Plotwise, I thought of the gremlins wanting to wake something up, then wreak havoc on a town in the process. then whatever they wake up would have to be an enemy for APL 5+.

Thoughts, suggestions, scrap it?

Use the "We be Goblins" as a base. Replace and reflavor creatures, etc. as needed.

Red caps can be nasty with a crit. Tell us how many PCs you one shot.