I have used my Paul Mitchell foaming pomade on top of using my John Frieda Relax serum to add shine. Thicker serums like KeraCare are good for both after styling and during styling (heat protectant). Thinner ones like Biosilk, Bain De Terre and some others can be used as a heat protectant, but b/c of their alcohol content are primarily used as a "glosser" or to add shine. Again, lots of people use Biosilk as a heat protectant (blowdrying, flat ironing) w/no adverse affects. There are a lot to choose from.
Serums are very versatile in their use. You should experiment to find out exactly which ones your hair likes. Try using a moisturizer/grease/pomade w/your choice of serum and see how your hair responds. I don't think you can really make a "mistake", per se, with them. They add shine, can straighten and/or soften your hair, depending on your usage.