What Matt Cutts said about Cross-Linking?
If you have multiple sites you want to link together you need to have a very good reason to be linking them together, otherwise it will come across as being spammy. Why do you have 20 domain names that you feel should be all cross-linked?
Cutts said about the only situation where this kind of solution is viable is if you’re linking multiple country site for the same company together. But even then, he suggested that you do it in a way that won’t come across as spamming. This means instead of having 30 footer links to all the different locations, find a smarter way to do this.
If you have 20 domain names and they’re all versions of the domain in different countries, such as Google.co.za, Google.fr, it can make a lot of sense to be able to get from one version of the domain to a different version. But even then I probably wouldn’t link all the domains all in the footer all by themselves because that’s a little bit strange.