OK, there are a lot of questions there, and a few different answers
Firstly, yes the Momo wheel WILL require it's correct boss.
And getting one for a Scoob is NOT easy. Try Alyn at ASPerformance, I'm pretty sure he knows a bit about it.
Airbag removal is not to be treated lightly. They can injure you if they go off, but if you disconnect the battery and wait half an hour, it's not likely to go off. As for leaving it off............
Have you thought about insurance? Your insurance company MAY not like the idea, but I've also heard of those who DID like it as it would save money in the event of an airbag-releasing collision
In any case, you WOULD need to inform them......
Fitting a smaller wheel , esp that small, isn't always a good idea. The car's steering rack and wheel are a bit like gears. Mess with the ratio and you could find it harder to steer.
Also, the control stalks are set in length for the larger wheel. If you fit one much smaller, they become a nuisance and harder to reach easily, or get in the way.
My lad has a Clio 197 with a very small wheel as standard. His car's stalks are the same size as my SONY steering column remote
Another option for you to consider is the fitting of a slightly smaller wheel from a newage STi. They look nice too

And retain the airbag, so your insurance won't care.
Alcazar