Does the minutes players play in the first scrimmage tomorrow impact the GS update on friday at all?
The simple answer is to just say no. Everyone is actually reset on friday. The interesting thing is whether minutes this week have any bearing on future game shape updates, since many people believe (and it should be noted; many don't) that the game shape calculation isn't based solely on the previous week's minutes. I've read that some think the calculation is actually based on three weeks' minutes, though where that comes from I can't really tell.
My personal belief (and its mostly a feeling), is that its a one week calculation, but the most important figure in the formula is the previous week's game shape, which misleads some into thinking that previous weeks' minutes have some bearing on later weeks. Therefore you would probably be safe to hold your starters out of the scrimmage, since it can't change the bearing this friday's game shape update (respectable for everyone) will have on next week's.
Having said that, I have so much faith in my own belief, that I'm actually using my starters as backups just to make up their minutes. I like to hedge my bets.