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Do you suppose it would do good to link them all together or would the search engines punish me harshly for that?
Do you suppose it would do any good SEOwise? The sites have pretty good original articles and the themes are all about the same (credit/home equity/debt/etc)
Your potential problem would be duplicate content and in the wake of all this scraper stuff this problem could well be exaggerated. It's fairly common knowledge that google will penalize sites with duplicate content (when they determine that that's what it is). I've been on the receiving end of this myself and can categorically state that it's true...
I had 5 sites basically disappear from google because of this. I knew it was a risk and took it anyway.
Lots of factors involved here. I said I had 5 sites vanish from google. I did. Not only did these sites contain duplicate content (my own) but they were all hosted on the same IP. Google looks at that, among other things. In my case I could have concealed my duplicate content. I could have renamed the downloads and rearranged my text and changed enough file names and content to appear unique to anyone who looked. I didn't do this. I was lazy. Plus back then I didn't know it mattered.
I have a company splash page with very little content on it that has a PR of 6 simply because I've linked to it from my other sites. PR is arguably a questionable commodity but this splash page was indexed by google within hours of my linking to it and now owns several niche type keywords and phrases, some of them over sites that are several years old and much more established than my little rinky dink splash page.
Your question was:
does it matter if the sites are related or not... if I link them would this help?
Help with what? It depends on what you're trying to accomplish. Ideally you want to get links from sites that are related to yours and have higher position than you and don't require a link back. In a perfect world this is what you would look for. It's not going to happen though so you look for the next best thing. In this case a link is better than no link, and I say this with some degree of reservation and the full understanding that all links are not created equal. There are always exceptions to this and some links should actively be avoided, shunned even.
To get them indexed? Yes. I mean if that's the type of help you're talking about then yes... absolutely. If that's a bad thing I'm hard pressed to see it...
If they are what I think they are then I wouldn't do it. Once one gets knocked out of the serps it will be that much easier to follow the links to your others and knock them out of the serps too.
As for any "real" sites you have I'd definitely link them. The more underhanded ones you want to stay under the radar as much as possible.