We need to link to the other wiki's more often. Isn't that what this is all about? Do we have to get permission for this, or is this something we can start right now?
I think as long as it's done a) with proper interwiki code (e.g. [[smashwiki:Master Hand|Master Hand]]) and b) with appropriate cause, there isn't much of a reason not to.
If another wiki covers a topic that you don't on yours but want to link to, why not?
There are also wiki's that have a sidebar system that includes links to the other wikis.