There is in many ways already a bandwagon server for NIWA wikis. Well, multiple really.
Fire Emblem Wiki, Wikirby, Lylat Wiki, and Wikibound are all hosted in conjunction.
Metroid Wiki and Strategy Wiki are hosted together.
Nintendo Wiki and F-Zero Wiki are hosted together.
Donkey Kong Wki, Super Mario Wiki, Smash Wiki, and Pikipedia/Pikmin Fanon are also hosted together.
lol, As you see a whole lot of NIWA are hosted on a server where another wiki is as well.
The only NIWA wikis that I think might consider this kind of a deal would be Starfy Wiki and Icaruspedia. Everyone else is pretty well situated.
Also, to answer your question, I would probably stick 3 wikis on a 1GB Linode, to be safe (Mediawiki recommends 256 mb per install)