I propose that S1 can record multiple bounced External Instrument tracks in the format the input is set for and not use up a whole Stereo track worth of data, just for a mono track.
I have several external synths connected to S1 5 using the new External instrument feature, some with Stereo inputs, the rest with Mono inputs. The different inputs are all configured correctly in the Audio Input page and the External Instruments properties.
When all the Midi parts are bounced to new tracks, The synths with Mono inputs are default bounced/recorded as a Stereo Track, even though S1 knows it is a mono input. If the track is then bounced to Mono afterwards, it's a louder summed track and then levels are too high.
I have an example below of a Mono External Instrument Track being bounced to Stereo by default by S1, then the extra gain when I have to manually bounced/converted to a mono track.