Dredging in cornstarch and then frying in a lower temperature oil is how you "velvetize" chicken. The cornstarch absorbs some of the steam as teh chicken cooks and seals in the moisture. This is how the strips fo chicken, pork, and beef are made so tender in Chinese restaurant buffets, and in dishes like chicken chow mein.
An alternate way of preparing your chicken for use with sweet & sour sauce is to cut boneless chicken into 1/2 inch cubes and stir-fry in hot peanut oil until just starting to brown. Imediatley remove from the heat and place cooked rice , chopped onion, and snow peas into your hot pan or wok, again with a little peanut oil. Stir-fry until the veggies just begin to soften. Season with soy sauce, a dash of 5-spice powder, and garlic, with maybe a bit of crushed red pepper if you want. Fold in the chicken and serve immediately. You could also add bias-sliced celery and bok choy to this dish, and even cashews or peanuts. It's very versatile. you could add scrambled egg, bias sliced carrot, baby corn, etc.
You could also make tempura batter by mixing 1/2 cup each all purpose flour and cornstarch, with 1/2 tsp. salt, and 2 tsp. baking powder. Add 3/4 cup water. Dip the chicken cubes into the batter and deep fry until golden.
Another yummy method, dredge chicken strips, or pieces through egg wash, flour, egg wash again, and then panko bread crumbs. Season with salt & pepper. Fry until golden in two inches of hot cooking oil.
Last idea, mix 1/2 cup shredded coconut to the panko bread crumbs and fry as above for coconut chicken. This is delicious and would work well with your sweet and sour sauce as well.
Seeeeeya; Goodweed of the North