@User Yup, I've designed chips that mix cells with different Vt, exactly for that purpose. High-speed on critical path, low-power elsewhere. Tools actually handled it a lot better than I expected. The question is more a nomenclature one than a teleological one. Certainly hs/ms/ls can and should be mixed within the same design. Just a question of whether they should sit inside the same folder. Not a big deal at all, not worth discussing that much. Whichever way it gets packaged is fine.

Were those tools you used commercial? I can’t tell if openroad can do the substitutions.

That is a very fair point. I keep forgetting, sorry. I get wrapped up in my own tool-flow.