They both modify the flevel. Flevel modding can't overlap. Kaldarasha and Sega Chief would need to combine their work into a single flevel to make them work together.
If I may add a detail, it's not just resize. Even if you switch off the resize option, if you activate New Threat (or New Threat Vanilla) the field models (or at least, the ninostyle field models, which are the ones I tried) will turn off, but weirdly enough the 7thHeaven won't warn you of any issue at all. Even more curious, it will specifically mess up the main characters models, but not, say, Jessie or Biggs. Cloud's face will also look deformed in is OG version. Absolutely no idea of why that happens, but I can confidently say that switching off the "resize" isn't solving it.
Other difficulty changes seem to do no harm to the field models, at least in the early game.
I don't know basically anything about modding, but it's a pity. Not that it detracts to the sheer awesomeness of this work, mind you.