Will This Hurt My Rank?

I'm really starting to LOVE using private networks for ranking, as I've been seeing a major increase in my ranking like I never seen before. These are all sites I'm building myself/built myself (some years ago), and I never realized how powerful an aged domain can be when you link it to a site you actually care about.

Compared to trying to backlink with new web2.0 sites, this completely destroys that imo.

But this is my question. I have access to some pretty decent sites that I built a while ago for my brother and his boss, and they don't really use the sites anymore. They have between a pr1-pr3 rank, some of the sites with a fair amount of backlinks. Some sites that are over 4 years old. They just continue to pay registration but aren't doing much with the sites. And I don't believe they will take them down anytime soon because they still use the sites in their business cards. They just don't care about updating them or anything.

So I figured, why not use the sites as part of my network?

And this is my question.

If I'm in a niche like mold removal, can I link to a site that does home remodeling? For example. One of the sites is a home remodeling site thats 4 years old, has built up a pr3, and I want to link to the home page. Plus maybe some of the backpages too. Then another site is an old waterproofing site, that has built up a nice pr3 as well.

The articles don't have mold removal keywords, so I'm wondering if I should change the articles, or add sections about mold removal? To make the pages more relevant before I link?

I just feel like I can get a lot of link juice if I do it right, but don't want to hurt my site. So how do I go about this exactly? Do I change the content on the home page to make it more relevant before I link? (like add a paragraph about mold or something?)

Thanks guys! - Red

ps. I already mentioned this to my brother and his boss (each have their own godaddy accounts) and they said "you built the sites so do whatever you want as long as our main services are still listed". I'm just curious if I have to add keywords to avoid any problems with the links not looking relevant.