You can use a 25.5” scale length for a 7 string but you can also use a 27” scale length for a little more stability. 

Just read about what the big boys are doing, they have worked it all all out. Just check out what is on the market and just follow their lead, they will have done all the testing. 

Here is a 25.5” scale length 7 string.

https://www.andertons.co.uk/guitar-dept/electric-guitars/7-8-9-string-guitars/7-string-guitars/ibanez-ictb721-bkf-iceman-limited-edition-iron-label-black-flat

Here is a great article about 7 string etc guitars. 

https://www.guitarworld.com/gear/everything-you-need-know-about-seven-and-eight-string-guitars

why would adding an additional string mean the scale length

@tej As far as I understand it, (and I could be totally wrong), the 7th String (the low B string) might flap around too much on a standard 24.75, 25, or 25.5 inch scale. So increasing the scale length maybe 27" is a way to avoid this from happening. 

The same way a baritone guitar has an increased scale length and the bass guitar (which is an extreme example). 

It might be fine, just something to think about. I would do some "window" shopping and see what other guitar makers do. I can see that the PRS SE Mark Holcomb SVN 7-String has a scale length of 26.5"

Edit: Thinking about it, to get the lower notes, the string has to be lose, which can lead to to flapping about on a standard scale. so you need the increased length to create the lower notes, but still need to create the right amount of tension to keep the string as tight as possible and not flap about. Maybe it'll be fine - we're not talking about large differences in size and I am not an expert (or anything close to that). 

I haven’t done a 7 string guitar but I have made two acoustic 8 string octave mandolas with a guitar scale length so it should be a reasonable comparison. I didn’t do anything extra to reinforce the neck on the first one but one the second one I made the volute on the back of the neck a bit bigger as the player wanted to use heavy gauge strings.

Mostly wondering if I need additional strength in the neck?

@tej I don’t have anything to offer for a 7 string as of yet because I haven’t built one. However, all the guitar building principles will be the same but maybe the string spacing is a little different, the neck dimensions too and the neck pocket. 

One thing about additional strength in the neck is installing some carbon fibre strips either side of the truss rod. I think there are other materials used also but that’s the general idea.