OFP: Resistance will only modify some of the official mission PBO files in OFP's MISSIONS and MPMISSIONS subdirectories. Other than that, OFP: Resistance's installation only adds new folders and files to your OFP installation.
Except as mentioned above, nothing else installed will be deleted or changed, including any unofficial addons or OFP version 1.5X or 1.60 beta, if you've installed it.