I have something similar. I'm with Hostgator, I have a VPS level 5. It comes with 4 IP address's and I have about 15 sites, some mine, some customer sites spread out over the addresses. There is very little interlinking between the sites but I was concerned too. I have read that Add-on sites are bad for SEO, but as long as you arent feature building crappy sites and linking them to your main site, should be fine.