I think the theory behind the scale length is that the longer the scale, the more the tention in the strings, and subsequently, the harder it is to play... However, with an added seventh string you might want to go a touch longer. I know Oskar Graf, who made guitars for the Tragically Hip and Don Ross uses a 26.0 inch scale to help with the lower string. I believe the one he built for the Tragiacally Hip dude (forget his name at the moment) was a 6 string but used for very low alternate tunnings (down to low C)... He also built a 7 string for Don Ross: Lots of info here on this :
http://web.ctsolutions.com/grafguitars/custom.html
Here's some info from Don Ross' site :
My main stage guitar these days is a 6-string custom long-scale guitar made by St. Thomas, Ontario luthier Marc Beneteau. It is tuned to standard concert pitch, but the extra scale length makes it much more capable of playing in tune while in one of my very low altered tunings. My 7-string guitar was a custom order made by Oskar Graf, an incredible luthier based in Clarendon, Ontario. It has an Engelmann spruce top and koa back and sides. The low string is usually tuned to the "A" a fifth below the low "E" (sixth) string and the highest string is usually tuned to the same "E" as on a regular 6-string. The string gauges run from .066" to .013". This is one of very few 7-string flat-top guitars anywhere. For more info about Oskar Graf, click here. The latest addition to the "family" (ack!) is another custom order: a Marc Beneteau baritone guitar! With an Engelmann spruce top and padouk back and sides, the guitar has an extra-long scale length and is strung low, like the 7-string, but with a tuning from "A" to "A," (i.e., down a fifth from a regular guitar). The string set gauges go from .066" to .017". Click here for more info about Marc Beneteau. What strings do I use? John Pearse medium gauge phosphor bronze (.056"-.013"). The 7-string and the baritone use a custom-made Pearse .066" string.
Hope this helps...
Alain