I assume you know that July is the rainy season. That said though, Guatamala doesn't have the glaciers as Mexico does nor is it as wet as say Costa Rica. It will probably rain every afternoon, but I guess if you don't mind it, climbing/hiking is still possible.
It's been a while, but I climbed Tacana and Tajumulco in the 1992-1993 season as well as visited the Lago Atitlan area. I would recommend them for hiking and the information is on SP in the form of mountain pages.