A little early to tell definitively, but your GLW and BR look to be a Pullet to me right now, but the BR has more white than I am used to seeing this early on, so keep an eye on the BR, unsure of the other as I don't have experience with that breed to give an opinion.
Never had an EE get so "pink" at 7 weeks, maybe a fast maturing Pullet, but I'd lean Cockerel 70/30 if I were a betting man, if "she" lays an egg I reserve the right to change my original opinion :)
I would not use unless you are experiencing issues within the flock, if you're feeding a good quality chick feed, that's all they need. We use a little sugar water or electrolyte the 1st day or so but nothing else but plain water after that.
Quality chick grower feeds will give your chicks the...
Color is throwing me off on this one, I think I see the start of male saddle's coming in, but I can't be sure. I'm 60/40 right now on this one being a cockerel, but boy I could be completely wrong too.
Depends on the age.
Comb and wattles are pretty red, and legs look rather large, so if this is a young bird, I'm leaning cockerel, and older bird would change my opinion most likely.
I agree, 6,7,10,15 all seem to lean cockerels if they are 8 weeks old, my experience is that the pullets do mature quick, but nothing with that size of comb and color at 8 weeks.