Generic Questions

[a_bertrand]

1) No the current combat module doesn't allow party of multiple players. But nothing prevent you to exchange the current combat module with something which fits more your needs.
2) If you make a module you will be the full owner of it. You can give it for free or sell it or do whatever you want. However making a module doesn't mean take one of the official module or a module developed by somebody else and modify 1 line of it. So if you take my own modules and modify them let me know it before you redistribute them and we shall discuss it.
3) Modules can accept other modules as extension. For example the clans do have hooks for it. Your own modules could offer hooks for further expansion.
4) You can of course make new modules which work in parallel of the existing modules or replace an existing one.