Your best bet is to call a local Key West tackle shop and explain what you want to do. They have guides on retainer/referral and they'll put you in touch with a guide who will do what you want (Inshore/Offshore/Reef...). Also, this way you're not chasing guides trying to find dates that match up.
Call Conchy Joe's Tackle in Key West and talk to Rob. He'll be glad to help you out.
http://www.conchyjoeskw.com .
BTW, not all guides will "guest captain" on a boat they don't know for safety reasons. They don't know if your bilge pump works, electronics, safety equipment, ... If you don't have the right safety equipment, expect the guide to say, "No thanks"; so ask if he/she has any pre-conditions.
PB