Well welcome to the world of reenacting! Oh and yes, by the sounds of it, I'd say the unit is "utter farb". Mosby's Rangers hit hard and fast, and its kinda hard to do that on foot, this reenacting unit of yours sounds like most of the "dismounted cav." units that permeate the hobby. For evidence I'd simply direct you towards the simplest internet search, Mosby's Rangers functioning as dismounted skirmishers is as crazy as the idea of them working alongside Confederate infantry, because simply put, they worked behind enemy lines, away from other Confederate troops on their own most of the time.
I commend you for your apparent good sense as you stopped and began to think. As for C.S. infantry units in California, I'm sorry I can't help you, California is a long way from East Texas for me to be of help in that department.
Good luck to you, and for more info on Mosby's Rangers, several members beside Mosby himself published memoirs, you can find them, and probably hear from them themselves on the notion of them fighting dismounted as skirmishers beside Confederate infantry.