after reading all that i come up with a few things...
"As a summer job" - it may be 3 months before you can sell them, and at a likely price of $15 per fish or less, you're not going to make a lot of money. Especially if you're new, as in not a known name breeder.
foods: Inforsia once free swimming for a week or so, then onto MW/ BBS and at another week or two maybe daphnia. MW are the cheapest as it's a live culture that is easy to care for so you can buy a starter really cheap. MW however can decay and affect the ventrals so i would not feed primarily. BBS are probably the most expensive, and care-intensive as you have to set up a couple hatcheries to hatch the eggs, but it's about the most effective once you have it going.
In addition to those there are also Vinegar eels, Walter worms, Banana worms, and grindal worms that you can buy cultures of and raise. They say worms make for bigger and faster growth.
Trying to limit the # of fry you are going to find difficult. she may drop 15 eggs after a single embrace, and you have to be lucky enough to catch them in the act, and then trying to grab her at that point may disturb the eggs/nest. Then there's the question of how many eggs dropped develop into wigglers, how many wigglers develop into fry, and then how many fry make it to adulthood. That's why they have so many eggs. Also you have no way to know that the 15 you let be born will have the qualities to make them worth selling/buying.
fry can live with each other up to 3 months, maybe more. it all depends on their temperament. Most of my spawn was jarred at 2 months. the runts still live together another month later.
Water changes... Size and health depend greatly upon this. I left the water for the first couple of weeks, after that it was 25% every 3 days or less trying to be careful not to suck fry up (you will anyway, it happens) after about a month i went up to 50% every 2 or 3 days.
Personally I was pretty lucky in my spawn. All made it to adulthood, and most of them are breeding quality (I don't think any except 3 of the 40 i've sold were for pets) and i've made probably just enough to recoup my initial investment, and not really going to be much over that when all is said and done.