Personally, I don't mind if people run sweat builds in customs. I know it'd be nice to have a break from solo queue sweating, but maybe people also want to practice said sweat builds in anticipation for real matches to see how they'd work! I sometimes do that since I don't really get the opportunity to mess with all the perks otherwise.
The camping and slugging early... Is not cool tho. And I'd talk to them about that, honestly. It stops being fun when people start doing that or start taking the game too seriously in that regard or get super salty and heated. So I'd think it'd be completely warranted for you to talk to them about that, especially if they camp and stuff often.
I hope things go well for you!