I know this is a late reply after your gaming weekend, but hope it helps anyway....
"is it possible to rent guns on the day?" - Depending on how many hires book for your day will depend on how many spare hire guns the site has. If they have enough left over they MAY be able to let you hire on the day but if they don't have enough then your left without a gun. My advice on this one would be to use your own guns (Assuming your Scar fails the Chrono, you will have your HK417 to fall back onto) and have your friends book a hire so no one if without a gun then
"would they be able to get their £15 refunded on the day?" - Probably not, when you book a hire you are basically taking up a "space" so if you then want a refund the site can't rent that "space" out to someone else as it's too late and thus makes a loss on that "space" (Hope that makes sense)
"are there any BB weights that aren't tolerated?" - There are no limitations to what you use, as long as you Chrono on 0.2s
"I intend to use 0.36g BB's in my pistols, is that alright?" - Yes, this is absolutely fine and not against any rules........but......(and this is a personal opinion)...........you may want to stick with 0.2s or even 0.25s in your pistol. Imagine your face on the receiving end of a 0.36 BB at a CQB range...it's going to hurt! So yeah...not breaking any rules, but maybe not the nicest thing to do
Hope this helps