15T = 15 teeth, you can guess the rest. You can run SS with the derailleur, so long as you can lock it in the right place to line the chain up with the sprocket (this can be bodged in a number of ways with or without retaining the shifter), but a chain tensioner in place of the derailleur is probably a better option - just to clarify, you'll need something to tension the chain as the dropouts in the tekiya won't allow you to tension the chain by moving the wheel in the frame. To remove the cassette you'll need a cassette tool and a spanner to turn it, and unless you want to do a creative bodge, a chain whip is essential. A good bike shop should help you out for next to nothing if you don't fancy doing it yourself. I've got the Kabuki Mono, which is the same bike essentially, but with single speed setup. I'm running a 22T at the front and a 16T at the back which works OK for me so far, but I think a 17 or 18 T would be a bit lower and better gearing (I'm pretty new to this game though!). I'm not sure what chain ring the Tekiya comes with.... When I got my bike the chain was prone to skipping. I shortened the chain by a link and it runs sweetly now. So make sure you adjust your chain so there's not much slack, i.e. so there's no way you can take out aother link without the chain being ridiculously tight. Hope this helps.