Well, everyone's hair is different as far as additives go, and you have to play around with them to see what your hair likes. You can put magnesium sulfate for curl enhancement, that is my favorite additive, but some people find it drying. I don't and I have dry hair. But it doesn't need alot added to be effective. You can add oil for moisture. Honey or agave as a humectant. Aloe vera and/or glycerin for frizz control. The possibilities are endless.
I use a hard hold gel over all my styling products, whether I'm using FSG, or another styler, because I need something to help my hair hold it's curl pattern without descending into a frizz pouf. I have a strong spiral curl pattern, but it has a strong tendency to frizz and I want it to spiral and not frizz. If a use a hard hold gel and then scrunch out the crunch when it's dry, I get really nice bouncy curls a la Shirley Temple
I like that.
My favorite hard hold gel is Aussie Instant Freeze. Which does not have glycerin in it. When I make my FSG, I put glycerin in that. I get the nicest curls when my FSG has glycerin but my 2nd gel does not - when both my gels have glycerin, it tends to be too much and I get flakes, plus my hair takes an age to dry.