I had a .70 as the 7th string on my 8 string guitar. First I would ask why you want a .70 gauge string? What do you want it to accomplish? I was so focused on the string gauge of the 8th string I wasn't aware the 7th was a .70; it became clear once I started playing though. Long story short a .70 gauge didn't distort very well, was hard on the left hand, and is "a lot of string for the pick to move." If you tune down it might be easier to use though.
Depends on your scale length. I use a .70 on my 25.5" COW7, its great for drop A and it is close to piano tight in B standard, but I also like at least 19 lbs of string tension. Go ahead and try it out, you won't know if it works for you until you do.
Guitar Strings Online recently added options to get DR Neons as a single string so I used a tension calculator to figure out as closed to balanced tension as I can get and 70 comes out to the best fit for B.
back when i had my dean avalanche ultra, i tried a 70. i had it tuned to b standard it was way too tight for my liking and too "thick/dull" sounding to my ears. tuning down to drop A fixed the tension but still didn't sound right for me. I ended up settling on a 64. the 64 worked well for me in both B standard and drop A.
After around 58 you start to lose a lot of clarity on a standard scale guitar. But to get to your question, I was once in a band where we used 15-70 on 6 strings for drop G#. We regretted it pretty quickly - ended up going with 12-56/58 - it seemed way too lose at first but after a few days you get used to it.
I use 58 for A on my 7th string - sounds great to me
Depends on why you want to use a .70. When I got my RGD7421 (26.5" scale) the previous owner used that gauge on the low B. its ok but I felt it too be to tight for standard B tuning. If you are down tuning maybe it will work. I use 9-54 now. its much nicer on the feel. Next string change i'll try a .60. Good luck!
I have a 70 on my Mayones Baritone 7 for Drop A, and it`s perfect.
The tension is comparable to a 48 low E String on a 25.5" guitar.
The other 6 strings are a standard 10-46 set.
Altough the step from 46 to 70 feels a bit strange at first, i got used to that very quickly.
You shouldn`t forget that we are comparing string size difference in hundredths of a millimeter, which is tiny even by a factor of 24.
It doesn`t feel as if you suddenly slapped a bass string on your guitar
I also tried 65 and 67, but they were too loose for my taste.
I should also mention that i like high tension, because it gives precision (in my opinion).