I am interested in BEADGC and minor and major thirds tuning
Try this
guitar nylon tapewound 7-string set by La Bella (discarding the topmost string of course):
"Bass VI" strings
La Bella 800-7 Jazz Tapes Black Nylon Tape and Stainless Steel Round Wound 7 String Electric Guitar Strings 14-79
Using it on your Squier Bass VI (I believe it would be long enough to clear the nut, based on my experience with a similar set by Galli on my own non-Fenderish, thru-body-strung bass VI -type instrument, but I cannot guarantee the highest three strings would reach their respective tuners, I'm afraid) you'd have:
- a plain top string (unavoidable - only Thomastik-Infeld makes a suitably thin tapewound treble string you could theoretically tune to high B on your VI: it's a high E from an odd, super-flexible metal-wound set for classical guitar, but it's not long enough for a 30" scale instrument - guess how I know...) and
- a non-tapewound low B, which La Bella in their wisdom decided would be a good match with the rest of the set. In case you end up not agreeing with them, you could replace it with a
single D'Addario bass tapewound .085
"Bass VI" strings
(You may have to thread it into a ~1.5"-long piece of thin tubing so that the ball-end sits away from the string anchor at the vibrato plate and, more importantly, the string tapers before the tuning post at the other end.)
It's either this, or getting the upthread-mentioned
La Bella specific set for the Bass VI
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discarding the low E and adding a single guitar plain high B. Assuming regular guitar strings are long enough, that is.
Final option, get more
D'Addario bass tape singles, beside the .085 (i.e. the .065, .05 and the .035), from the link above, and add two single strings, again hoping they're long enough. (Something like .016~018 plain or .017~02 wound - "regularwound", that is, not nylon - for the F#, and .012~014 for the high B.)
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As regards major-third tuning, I'd use the same set and choose which exact tuning (I mean, from where to where) so that either of the two middle strings of the set maintains its pitch, and the top and bottom ones get detuned but not excessively so.
As for tuning in minor thirds, I'd get
another an extra set of
(light) La Bella guitar tapes, the 800-L
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La Bella Black Nylon Tape Wound Electric Guitar Strings
and mix and match gauges [
with those in the 7-string set; or alternatives thereto, as detailed above, hoping tone across strings doesn't prove too patchy] until you get similar perceived tautness for all strings.
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If the weird string length requirements of the instrument (over 34.25" but less than 35.75" for the lowest, as long as 40.5"+ for the highest) get maddening, consider swapping the vibrato plate with a hardtail one, with anchoring holes set a little forward:
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If you go for it, a D'Addario
medium scale .085 (and of course no short tubing tricks)
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would be a better fit for the lowest string.