I have a few sites and they are some what related. I was told not to post my links from each site on to the other site, but can I just post 1 link on the index page over to the other site ?
I have posted a link from one of my sites to another and have had no penalties. I also put our "designed and maintained by" link in the footer of each website we desgin and have - so far - had no penalties. I have, however, a similar question to yours - I have a customer with one existing charity site and two business sites about to go live - can I link from his charity site to the business sites? They are not related in topics but his businesses will be raising money for his charity. The charity site is doing well in the serps as the search terms are non competitive; the business sites will need all the links they can get.
Are the sites on the same server and on the same IP address? If so, you might want to consider moving one of the sites to a different IP address. A little interlinking is ok. But dont go overboard on it. I maintain a forum for my wife and my son. To target some keywords, I added a link in my signature on my sons forum going to my wifes forum. The next day traffic dropped - and it continued to drop for 2 weeks until I removed the link in my signature. Not only did traffic drop, but the sites lost their ranking in certain keywords. The whole situation was bad. That was about 6 weeks ago, and the sites still have not recovered they traffic they had. Just so you will know, the signature in my sons forum gave my wifes forum about 600 - 700 signature links. My answer is - a little interlinking is ok. But dont go overboard on it. If you do want to build links, build them slowly. On the other hand, I have a blog and a forum that has a massive amount of interlinking. The forum has about 7,000 links going back to my blog. The blog has a few dozen going back to the forum - and both sites rank very well in their targeted keywords. But I built the links over a long period of time.
You can add as many links as you want with no problem, provided you use the NOFOLLOW attribute in your links.
Yes, its fine...just don't be excessive. You may not get a huge benefit SEO-wise if they are on the same IP C-block, but it still may drive some visitors.