I needed to bulk up on muscle mass on my legs
Absolutely fine, so long as the supplementation is solely DIETARY.
Please do not be tempted to get involved with anabolic steroids, or anything like that, as some bent athletes do. They might increase muscle mass and strength in the short term, but at a great physiological and pathological cost in the longer term.
may incur negative urine sample results if I were to go for my class 1 initial
Same applies here too. The taking of most types of steroids ( not inhalers ! ) is usually disqualifying for a class 1 medical. But what you take in as DIET, and how unusual or expensive it is, will not affect a class 1 medical one iota.