The GSM level 37 skill is not an affinity skill, but it's so hilariously bad that you'll never want to put it on your crosshotbar anyway. Flawless Synthesis is a skill that increases your synth progress by 40. It costs 15 CP to use, and has a 90% success rate. By the time you get it, though, unless your equipment is ludicrously bad or you're attempting a recipe way higher than your level, your Basic Touches (which cost NO CP) will be doing nearly that much, and after just a couple levels will be doing MORE than that much.

It's frankly bizarre that SE hasn't done something to improve this skill, because as it is it is pretty much unusable.

Anyway, as was stated, all level 15 skills should fit on one crosshotbar. Since some of your classes are not yet level 35, though, you won't be able to set all of your level 15 skills at once. If you had to pick one level 15 skill to leave out, probably Rapid Synthesis would be a good choice. You have to be pretty desperate to rely on that skill to finish a synth, though it does have its moments.