Hi Mike, fellow Soul owner here and I’m running a 120mm Reba and find it’s a good compromise all round. If your main interest is climbing and bombing through the woods etc. then 100mm would do the job fine, however as you’ve got the RL for that then you’d be better off going for a longer fork to increase the difference between the two. The handling is still pretty sharp with a 120, it won’t climb as well but will descend better.
As for adjusting fork travel, most aftermarket Rebas can be adjusted to 120mm but some supplied as OEM forks are fixed at 100 so unless you can confirm before you buy that they are adjustable, I would say avoid 100mm forks.
The difference between SLX and XT is pretty minimal, I have SLX on one bike and XT on the other. The XT shifters are slightly crisper in their action but the main difference IMO is the drag from the jockey wheels. SLX runs on bushings and XT on bearings and when you compare them side by side there is a difference, however this is easily cured as jockeys are a consumable item and easy to fit a set that run on bearings when the originals wear out.
I’d say put the new gear on the Soul as that’s likely to become your main bike once you get it built up !