Only 380s planned for Barrhead electric services at the moment. Apart from Dumfries and Stranraer drivers, all other drivers that sign Barrhead sign the 380, as do all Glasgow, and soon to be Ayr, Conductors. Don't think they actually have any "spare" 385s to be used on Barrheads, as they only have one 380 available for the December timetable change.
No idea what the plan for East Kilbride will be unit wise, as there's almost no chance that ScotRail will have taken delivery of any new EMUs by then, so other routes that 380/385s run on will likely be short formed to allow an EMU service on EK line.