I wore a Vans shirt today while wearing Cons. Hate away y'all.
....But seriously, intermingling brands has never bothered me too much. I have a lot of skate related OCDs, but that was never one that fucked with me too much.
As for stickers, I'll usually put a few I have lying around on my setups. I just slap em on wherever they might look good. I make sure not to overdo it with the stickers though. An over-stickered deck just doesn't look good to me.