Shockingly, this has come up before, see Thoughts on Mutation
Tl;Dr: more extensive fitting is fine, a number of JSON flags have been added to the system to facilitate this kind of thing, and most of the low-hanging fruit has already been “picked”.
What is left is mostly clothing that would be partially compromised by being altered to fit various mutated anatomies. See Thoughts on Mutation for an outline of what we could support going forward. Basically, mutations need entries indicating how clothing fitted for them would be impacted, (i.e. reduced protection, coverage, warmth, storage and/or increased encumbrance). After re fitting, an article of clothing would have those penalties attacked, as well as flags indicating that they now can fit over various mutated anatomies.
Have fun.