I have a question, how much of a gap from the top of the stem to where the steerer tube starts is okay/safe? The tube is carbon and the stem is alu.

As long as the steerer is higher than the top bolt on the clamp (back of the stem) then I would ride it with a bung like this that sits at the top of the steerer.



Ideally the steerer should be cut above the stem with a spacer on top imo.

Also don't expect the fork manufacturer to sign off on using a fork with the steerer cut below the stem unless it's Cannondale, and in that case don't expect them to sign off on using a fork with a steerer cut above the stem.

I would agree with bricky hat as long as you don't cut below the level of the stem clamp bolt you should be ok. Spacer on top with steerer cut 2mm below that is the best option though.