If a solid itchy red bump weeps clear fluid from time to time or from rubbing or scratching, does that mean its infected with staph?

No, it is probably not. A vesicle on top of a rash due to insect bite such as mosquito bite can re-accumulate after you scratch it open. Of course, when the skin is broken, natural barrier is impaired, chance of skin bacteria such as Staph can cause superinfection but it is a limited condition and the lesion will heal eventually if the site is kept clean.