Robbie,
the problem is the inside of tubeless tyres are not designed for tubes. As I understand it they can cause punctures. My spare leaked and after the process you described I added gloop. It is now fine.
Depends how low profile the tyre is. If you are reasonably close to 80% aspect ratio (i.e. standard) then heavy duty tubes (Michelin) or tubes of the same manufacturer (Vredestein in my case) do not suffer problems. However, I wouldn't fit a tube to a tyre of less than 60% profile.
