Can I switch formula brands? My baby is on similac, but I got enfimil and gerber brand for free. They are all the normal formula.

Maybe. Although all are required to meet the same nutrient minimums formulas are not interchangeable. Switching out may not bother some infants but can lead to gasiness, constipation or colicky symptoms. Some stores will let you trade out formulas on a can for can basis. Beware, some freebies are worth what they cost.
Sure. I see no reason why you couldn't switch, as long as baby continues to feed and gain weight appropriately.