Thanks Dutret.

Hmm, so if I were to get an Omnium, I would be stuck with whatever BB it came with (105mm length) and I wouldn't be able to detach the cranks to switch them onto a 128mm bb?

Okay so I guess mountain it is. But, what happens with the space in the BB for the derailleur attachment which I won't have, a spacer I guess?
Also, the chainline measurement that mountain cranks will give, is that for the middle chainring? What should I add to that to get the outside chainring which is where I'll attach a 1/8" one?