Yeah Fumoffu is the pure humor side of Full Metal Panic taken to 11. I wouldn't necessarily say that it would ruin your viewing of Full Metal Panic, so long as you know that you'll still get some of that Fumoffu humor, but that there is a story to go along there. Full Metal Panic has four major arcs that are divided by those comedic moments you are more or less used to seeing. Second Raid is a direct continuation of Full Metal Panic, a sequel to the story started in the original. It more or less is just one arc, but is a little one the looser side of things than the more rigidly formed arcs of the original. It does end up finishing up with solid arc.

Full Metal Panic is really good if you're looking for great mecha action along with the standard type of actions, military organizations and a good dose of high school comedy brought to you from Sosuke, who is simply amazing as a straight man that does stupid and funny things.