On a 1911, the staked front sight can be tricky - it can go south really fast if you're not careful - so I'd recommend smith installation for that. There are even special tools out there for it, if that's any indication as to what's involved.
Also, I recall the XS has a few different dovetail sizes available for 1911's - if you can't find an exact match, go for a little bigger and the same smith should be able to make them fit, provided that the angles are the same.
If it's a box-stock Defender slide, and XS doesn't list it specifically, give em a call and they should be able to help you figure it out.
All the XS sights I've gotten have been slightly oversize and required minor fitting, but that's to be expected. Overall, their QC seems to be pretty good - and it's better to have extra meat to work with than not enough..