We've never had a problem keeping it a pro-Metro crowd for games against Plastic City. Most of them won't travel "all the way" to NJ to support their club.
I used to have seats in 218 until last season - NYCFC have had the majority of the second tier of the "North Ward" (217-219) in addition to the 220 visiting section at all of their MLS games at RBA (they traveled poorly for the USOC game... we won 4-0 so a good decision), spilling into the corner sections. Now, most of those seats are tarped off and everyone will have to find new locations.
It's going to be a pro-Metro crowd overall, but there are still usually thousands of NYC fans making the trip, which doesn't really happen for any other away teams at RBA. Some advance planning by the FO is in order. Removing the tarps is one good option. Doing nothing and hoping for the best... isn't.
Reducing the capacity by 3-5k+ may actually result in a full stadium on Sunday... in the worst game for it. The two sets of fans don't like each other, and there will be plenty of people drinking all day in advance. It's a recipe for disaster.