Accessing the other sites through subdomains is an odd setup. I've never seen that before. I wouldn't link to sites served up on subdomains of other domains. Even so, googlebot will probably discover those eventually. That might cause some duplicate content type of problems. It might be worth shutting down that functionality or mitigating it with url canonicalization.
In general, having several sites with different focus on the same host should be fine for SEO. Google allows a single webmaster to own and rank for multiple sites that are not targeting the same keywords.
The down side is that if you run afoul of Google's guidelines with one site, Google may penalize all your sites. Hosting is one of the signals that Google probably uses to determine site ownership. Google also uses domain registration, interlinking profiles, common templates, webmaster tools logins, and adsense account information to figure out which sites are owned by the same person. Folks here have reported losing traffic to all their sites at once when Google puts a penalty in place.