Standard tuning, because it's a result of string tension.
I've used 25.5" in drop G with absolutely no problems with a .070 on the 7th. I prefer longer, but there's no reason you can't use that in B or A. I used a 6er in those tunings for ages.
yeah on a 7 25.5 is fine because your really only bothered about the bottom string and you can just up the guage.
I got an 8 recently which is fanned and that addresses the brittle treble strings quite nicely and doesnt take much, if any, getting used to.
Maybe worth a look if you want longer scale on the bottom end.
It's all abobut personal preference at the end of the day. I've had longer scale guitars, but I always gravitate back to 25.5" scales. I'm currently in drop B on a 6, but the band has previously been down to drop A. My bandmate uses a 26.5" scale, just because that works better for him. Go try a couple out and see what works for you.
I like 26 inch scale.
I've had a 25 - 26 inch scale 7 and I hated the feeling of the lower strings so much that I trashed the design and moved it to 26-27.
25.5 is pretty much the standard for 7 strings with 27 being the most common alternative unless you buy a Schecter. There's more than enough brutal super low tuned metal being made and having been made with downtuned 25.5 guitars so pretty sure you're not gonna have an issue with it.