on my SS street/park/jumps bike, i found that running tensioners on the droputs was enough, as long as the chain line is ok you should be fine, i've never dropped a chain so long as i've had it. With regards to the disc, i think as long as you're sensible you should be ok, and maybe get a half link chain so that you can make reasonably fine adjustments to the chain length. That's what i'm running right now, and as i say, runs absolutely perfectly, nice tight chain so no chain slap and i've not dropped the chain once. Oh and that's on a QR rear axle! bit of a bodge but it works fine!