Every costume that you own will have its bonus active at all times, no matter which costume is currently being worn. Costumes are particularly powerful because of the way their bonus is calculated in comparison to other profit multipliers.
Typically profit bonuses are multiplied by the business’s base income. However, costume bonuses are applied after all other calculations are done, even the ad bonus! This means that even if the costume multiplier looks small, it has a massive effect.
So if we assume a business’s base income is 1, then:
(1 x [character bonuses + business bonuses] x ad bonus) x (sum of all costume multipliers for that particular business)
Now go get those costumes!