It depends how cold your house is. We live in Florida but even though it's hot outside we kept the kids in footed sleepers because it gets chilly in the house. I think we kept Grant in the footed ones until he was about a year old and used a blanket. He couldn't really keep the blanket on but he's kind of hot natured so he didn't want his feet covered anyway.
Like PP said it depends on your house. In the summer we keep ours set at 68-69, so I will be keeping DD2 in footie jammies. But, with my FB, she was a naturally warmer kid, she would get heat rash unless she slept in just a diaper after about 4 months old during the summer months. So, I guess it also depends on the kid.
Re: Summer clothing