Well, I think of it like this:
Having seperate Wikis improves searchability. For example, people will type "sfh2 wiki" into google. Because of the intuitive search, it may lead to the SFH 1 Wiki instead. This may lead to people not finding the correct info.
However, I am all for having interlinked Wikis. I have made sure that we are linked to the SFH 1 Wiki, but I do agree that it is not really enough.
I would really like there to be a SKY9 Games Wiki, with all the seperate game Wikis "nested" within it. This way, they all could retain their own management, while still being in the "same place". I do not think that a series should have just one Wiki, as the two SFH games are very different, for example.
In conclusion, I think all Wikis should be in the same place, but not on the same Wiki. The benefits of making the pages more clearly linked would be that a reader could read about other games too. However, I believe that the Wiki boundaries should be clearly marked.
And that is my opinion. :)