What I find amazing is the massive assumptions you are making in your post. You assume people who where giving him advice did not understand what child support is. I saw no indication that anyone other than the original poster was ignorant of what it entails. Also, you display a bias towards men in that you assume those who want to acquire custody of their children only desire to do so to avoid child support. From many of the situations I have seen notated in various forms of media, those who take that path are a small minority. To essentially disparage men's reasoning to gain custody of their children in the manner you have, exposes a bit of a misandrist bias.Yep, they ain't trying to hear about those Pampers you bought that time..LOL
It's amazing what people think constitutes Child Support. And, because of their misconceptions, when they get their actual orders they feel cheated. And, that's when they decide they want custody.
The OP says that he has enough money to pay Mom's college tuition, and support her and the two children. So, he will be paying much more than $500/month, imho.
Last bits of advice for the brother who asked. Learn to wait until you are married to have children, wear a condom before that point no matter what. Also, If you think you may want more children in the future, have some of your sperm stored and get a vasectomy (Even with the vasectomy always wear a condom when not married). Too many of our people allow emotions (Lust in this case) to override good sense, and so make poor decisions that have long lasting consequences. Since you are now enlightened to how shortsighted decision making can lead to long term struggles, maybe learn to hedge your bets and protect yourself from now on.
Last, focus on paying your child support, finishing Nursing school, and creating a home environment that will be conducive to raising well adjusted young ladies. You can not control what the mother does, so your main focus should be on doing what is in the best interest of your daughters. Doing the above three I mentioned will go a long way in helping your case for full custody of your girls.