Do you have specs for the ballast? Like a name and model number? I would bet it depends on the ballast if it can only run one bulb.
175w wouldnt be too much for a 29 gallon. There are several people on here that run 150 over their 29 gal tanks.
Ballasts like every other piece of equipment will die. It could be tommarow or 10 years from now. But you would have to look at the manufacture specs to see what they consider the life expectancy of that paticular ballast.
The track that the legs slide into goes the entire length of the fixture so you can slide the legs far enough in to fit your tank. I've got that fixture in a 48" and while it's not the highest quality I have nothing bad to say about it, it does exactly what it's supposed to.
I've been told there's no such thing as too much light ... And if you really wanted to cut it down you could run the PCs just for the morning/dusk and just have the MH on during the day to cut it down to only 250w.