I'd create two separate registration path, assigning the two reg types that are unlimited to one path and one for the one you want to cap. For the one you want to cap, enable waitlist for that path, and make sure to assign a limit to the number of folks who can select that registration type. This should allow registration to remain open, without forcing you to cap the total for the whole event, while shutting down registration and waitlisting the folks of that one type.
------------------------------
Tony Odett
Manager, AMS and Registration
Edison Electric InstituteUnited States
------------------------------