A couple of questions about mapmaking.

I searched everywhere but couldn't find the information I needed. Neither here on the forum nor on Google. I want to learn how to edit random map templates. I'm willing to spend time learning.
And here's a question: is it possible to remove heroes/cities from the list of available ones by editing the template? And if not, what method can be used?
If this question shouldn't be asked here, please share a link to where I can ask it.