FYI, corn starch is a starch made from corn, much like flour is made from wheat. It's natural and as healthy as any other starch/thickening agent.
When making a stir fry, take the tofu and drain. I use a small holed colander or steamer, put the tofu in, then a small plate on top and a heavy can on top and leave it in the sink to drain. I don't bother with the paper towels. Just before browning, I cube the tofu, then dip in water and then dredge in cornstarch. Then pan fry (you don't need much oil for this) until you have a nice crispy golden brown coating. The tofu is then ready to add to the stir fry. You don't want to marinate the tofu. While you want to marinate meats for stir fries so that the meat absorbs the flavors, you don't do this with tofu. The skin will hold the tofu together and give it a good texture much like something that was breaded and deep fried, but much lighter. When you stir fry add the tofu, the cornstarch will absorb a small amount of the flavors and lightly season the tofu. Any more than that and you will soften the skin and you lose the texture (the point of the dredging and pan frying). It will taste fine without marinating. Just make sure to get the right favor in the stir fry sauce.