So wait, does this mean I need to change something other than what i2amroy mentioned?
No, what i2amroy suggested will work perfectly.
Ah...
Wait, what does ARMORLEVEL do then?
It's the same tag, just a different argument. In my experience, you can typically expect a civ to have something that's COMMON, but to be on the safe side you can make it FORCED instead.
ARMORLEVEL, as I said, has few effects. In 40d and before, it determined which of the 3 armor sets (leather, chain, and plate) an armor piece was treated as part of and thus whether a dwarf set to use a particular set would put it on. With 31.01, the armor and uniform system was totally overhauled and now the concept of armor sets no longer exists.
The only thing it now does is interact with the entity-level armor assignment tokens. At the COMMON rarity level, a civilization generated off of an entity will have at least one form of covering for each body part for each ARMORLEVEL. Low and high boots are both ARMORLEVEL:1, they both cover the feet, and they both are at the COMMON rarity level for the dwarf entity. As such, all dwarf civilizations will always have at least one of low boots and high boots, with the possibility for both. As shoes are ARMORLEVEL:0 (actually unassigned, but defaults to that), they don't interfere with nor are interfered with by boots.
At the FORCED rarity level, a civilization generated will always have that particular armor piece; if you changed nothing but giving high boots FORCED rarity for the MOUNTAIN entity, you'd always get high boots, but would only sometimes get low boots. If all armor pieces for a body part at a given ARMORLEVEL the UNCOMMON and RARE rarity levels, I believe that it's no longer certain that you'll get one for each body part. I think that the LAYER token might affect this interaction as well, but I don't know that for certain (I don't recall if I've ever seen a civ that had only one of shoes and socks available, which would clinch it).