Pro Tools | First supports mono, multi-mono, and stereo plug-ins.

Because inserts process in series, changing the plug-in format can alter the channel format. For example, inserting a mono-to-stereo plug-in on a mono Auxiliary Input changes the signal path from that plug-in through the rest of the track. This restricts all inserts after the stereo plug-in to be stereo-in/stereo-out plug-ins.