26.25" seems a bit short for a baritone (tuned B to B ), but may be okay for your mandocello.
I have two electric baritones and one acoustic.
On one electric (27" scale), I have 12, 16, 24p, 32, 44, 56, which is really noodley.
On the other (again, 27"), I'm using whatever came stock on it (just got it a couple of weeks ago -- an ESP LTD MHB-400. I think they need more letter-number combinations.)
On the acoustic, I use 16, 22, 29, 48, 60, 70 which is a bit tight
It sounds like an interesting project, keep us posted