I've heard that even though it's hot a new born has issues regulating heat, so to use a light cotton long sleeve sleeper at first especially at night. Then to use onsies and layer. Just keep an eye on their hands and feet and to cover them if they're cold. Also, try and stay inside during the peak heats of the day. July is hot and dry where I'm from.
Our summers are usually in the 100's and we live by the water so it's humid all year round. I'll only be having his newborn clothes as short sleeve and must of his 0-3 months as well. If he's cold I'll be swaddling him. I did get a few sleep and plays for 0-3 months that are of course long sleeve. That's just what I'll be doing. Oh and due 7/4/15.