I don't know about the night bus but I do know it's a hell of a long way and I definitely wouldn't want to do it in the dark. if you've got time I would take a day bus to Kabale, hang out there for a night (or go out to Lake Bunyoni) and then get a mini-bus taxi to Kisoro.
Whatever you choose to do allow plenty of time as the bus won't leave until it is full even if they do tell you a departure time.
Alternatively, you could get the post bus to Kabale (if one goes there - check at the Kampala central post office) as you can buy tickets in advance and it leaves at a scheduled time.
On the subject of post I remember an amusing story about Kisoro. A friend was living there and a had a visitor from the UK. They were walking along main street one day when a mini-bus taxi flew past and an envelope was thrown out of the window. My friend's visitor picked it up, looked at the name on it, turned to my friend and said, 'It's for you!' And it was. Somebody in Kabale had asked the driver to deliver it and he just threw it at the first white person he saw.