Absolutely not. I can state with 100% certainty that the red bird in the pictures is not a cockerel of Red Sexlink, Gold Sexlink, Gold Comet, ISA Brown, Red Star, or any of the similar kinds.
The whole point of those sexlinks is that the males are a different color than the females. The females are red, and the males are white. This is more obvious when they are young chicks, and both sexes usually have some of each color by the time they are grown up, but they NEVER have a male that is red with bits of black and no white at all.
A Rhode Island Red, or a New Hampshire, or a Production Red, would have males and females that are red with a bit of black in their tail. I think the male is probably one of those. They look enough alike that there is no way to be positive which one he is, considering the general quality of birds from most hatcheries (the fine points that distinguish show-quality Rhode Island Reds from show-quality New Hampshires are not reliably present in hatchery flocks.)
I agree that seems likely.
Yes, that is always a good idea when trying to identify breeds, although these look fairly obvious to me.
Those chicks are probably big enough to eat pellets by now. They shouldn't have layer feed, but the size of the pellets is not the reason.