For the record regarding the first post - the date when my friends were looking for a venue was 5th November, when we are at home to Hearts. RA checked out whether it was feasible to take a booking for a 21st party after hospitality was over and we concluded there would not be enough time to get the room ready (I think hospitality was looking like it was going to be busier than usual for the Hearts game). RA offered us an alternative date and was also open to the room being partitioned to suit the size of the booking. In the end my friends found a venue which was able to accommodate them on 5th November.
Also, for the record, RA on behalf of St Mirren was a lot quicker to reply and was a lot more helpful than a lot of other venues in Renfrewshire: hotels, sports clubs, village halls etc that were contacted about this party. I would definitely look at booking facilities at St Mirren Park in future based on my dealings with RA on this occasion.