You can tell if it affects shifting by riding the bike. Probably it won't, unless the cable (not the housing, but the cable inside it) is exposed there and rubs.
You'll have to work out the light/bag relationship on your own. I'd probably use a small handlebar bag, but you might also attach the light to a chain stay or attach the pouch to the front end of the top tube with (perhaps) a Velcro strap.