For me, nothing stops you from using your ISPC in creating emails for your public websites. So, basically you got nothing to worry about its LE SSL files as it will use your ISPC LE SSL files. About your subdomains, that depends on how you gonna use them. If you need them as separate sites, use subdomains (vhost) in there you can create and manage LE SSL files for them separately. If you simply use * in main site, and create simple subdomains, ISPC should create an LE SSL files for that subdomains too, except they are "bonded" with the main site.