I have ZipFit liners (Plug Leather from last season) and lots of Flexons. I have attempted a couple times to put the plug leather liners in Flexons, but unfortunately they take up too much volume and I can't get the buckles to close. I even moved the cables in their "anchors" to make them longer and it wasn't enough help. You might be able to do it if you can get longer buckle cables.
It's interesting to note that these ZipFit liners fit perfectly into my 306mm shell Kryptons, but can't even come close to getting in my 301mm shell Flexons (of course this is mostly due to the cuff and ankle area, not the lower foot portion of the liner).
Now which boot do you think is lower volume overall?