As I see it back links can never hurt a site if that was true people would run out a point loads of bad links to rival websites. As far as links from the same IP, I personal think that the search engines put a devaluation on each additional link from a website point to the same place. So that each link after the first one carries less and less weight behind it, so that people can just get loads of link from the same place because eventually they will be worthless. any link building campaign should gain links from as many different websites on different IP as possible.
I would like to think that search engines such as Google will penalize this. Remember crawlers have common sense and think very logically, especially those at Google and Yahoo. "So, why should many idential links come from the same IP", crawlers will think and will use logic.
I don't think it's the IP that matters, more the site name, and how reputable it is in the search engine already.
I think the best word to describe this would be DEATH The main site will go to a PR1 and the ballance will go to PR0. This is call "sub-net linking" or "sub-net cross linking". I was done about 5 years ago and everyone was killed. Don't do it. With that said, get all site up and running correctly without cross linking and ask us again. You could have fun.
backlinks cannot hurt you but at worse they can have zero positive per google, but remember, you could gain alot of traffic from that backlink even if google gives zero benefit for that link.
Have you looked into neurolinker yet? Just came out couple of days ago. It gives you lots of one way inbound links from multiple sites and you can accept or reject them. Most have PR. Its very clever. neurolinker.org to check it out.
Same IP is reduce your site value by google. I got effect. My site decrease value due to same IP linking.