To get a site spidered and indexed, yes, this is a good thing. It's been years since I submitted a site to any search engine. Just link to it from my other sites and the search engines will pick it up, often w/in days...
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.
does it matter if the sites are related or not? what Im asking if is I have a site that is music related, other that is pictures related and other site that is ebook related, if I link them would this help?
If you have a site that has already been indexed by some of the major search engines and gets spidered even halfway regularly and one day you launch a new site, the smartest thing you can do for yourself - bar NONE - (short of buying your position), is link your main site to your newly launched. I don't care if it's related or not... at this point that has nothing to do with what you're trying to accomplish. It's a new site and you need to get it recognized. This is a proven shortcut to doing that. I've launched new sites that literally showed up within days in google serps. If you're interested in doing that then by all means link.
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...