As BensGrandad says, the U-21s/development squad/reserves are all one-and-the-same thing. We have already had a couple of trialists play for the U-21s this season (Kieran Sadlier and Chike Kandi).
Quite a few (but most of them will be played at Culver Road, Lancing, and nearly all of them are scheduled for Monday night) - U-21 fixture list from the official site