I've run 24g singles in my Mutation X v3 before. I've never been huge on singles, but I remember something between .3 and .4 being better than I expected. A .2-ish single parallel 24 is pretty warm and tasty. I will always recommend those for a full-on, sub-ohm single-coil vape.
Dual 24's are my favorite mech builds for both the v3 and the v4. I consider them to be sort of the benchmarks for your traditional, dual coil, sub-ohm vape. They're the definitive, high-powered builds that I think everyone should try at some point. They're nothing fancy... ...they just tick all of the boxes and always satisfy. When I'm not sure what to build, I do a dual 24g.
I find that for mechs, 2.5mm .18-.24 dual 24's fire-up real nicely and provide much better flavor, more warmth, and slightly better vapor production than dual 26g builds. The lower range is warmer and cloudier, while the upper range is cooler and tastier.
For regulated devices, I go just a little higher in resistance and diameter than I would on a mech. I usually like to be somewhere in the ballpark of .26 and .32 @ 3mm.
26g is still great to use for duals in the Mutation family, though standard dual 26g builds tend to be a bit lackluster ime. Ever try running dual-parallel 26's in one? Twisted 26's are pretty legit, too. I almost like those builds more than standard dual 24's.