CMSMS Core allows removing the links and any reference to CMSMS in the admin theme.
CMSMS Core allows removing the links and my reference to CMSMS in the frontend
You may NOT change the copyrights in the source files in the GPL version. You must purchase a license for that.
Third party modules may add additional license restrictions, so you need to check the license of EACH and EVERY addon module.
For example, ALL of my modules have this additional requirement:
However, as a special exception to the GPL, this software is distributed as an addon module to CMS Made Simple. You may not use this software in any Non GPL version of CMS Made simple, or in any version of CMS Made simple that does not indicate clearly and obviously in its admin section that the site was built with CMS Made simple.