I care a lot about realism. If something I put in my universe is something that could not physically exist, it will bug me until I have somehow justified it or removed it. But when creating a fictional universe, realism is flexible. There is no reason that another (even real) universe can't have different constants that cause elements to have different properties, and for particles to behave differently. But anyway, assuming our universe's rules:

So, the higher the melting point of the material at the core of the mountain, the higher it can become? Melting points are pretty variable between different elements, so if for some reason large deposits of rare metals accumulated there, the max height of the mountain could be a lot higher?

I wonder on what timescale mountain ranges "melt away" using those processes. Giant mountains could still be young ones.