So, I had a cool orange gas pipe frame, and a pair of bullhorns. I wanted to do something fun with them, and I was bored, so I hauled out an MTB with relatively narrow (for an MTB) hybrid style rims that someone gave me, and started making an urban assault bike. My idea, have a bike with enough rim width for a bit of trail riding, blasting across parks, hopping curbs etc, but with the riding position and geometry I'm used to, plus, to try bullhorns. It cost me nothing, so it's a beater I can lock up and leave, with the fenders, I can use it in the rain, ice, snow. I'll add a rack, so I can use it for commutes etc too.
So, I've got a ghetto single ring crankset (someone had done it with bolts and nylock nuts, rather than pay for single ring bolts! Perfect for this bike), so all I need is a cassette convertor kit for single speed. I'm such a C&V grouch that I've never messed with a cassette hub before, and I have a question. The kits seem to come with 16 tooth sprockets, but I think I want 42x15, so what I'm wondering is, can I grab a 15t sprocket off the cassette I'm removing, and use that instead?