Hi, I can't help with the string gauges, I'm a "suck it and see" kinda guy. But you mention guitar and uke, are you stringing with steel strings as a guitar, or nylon strings, as with a uke? it may help others with their answer.
Don't forget HTML...
I’ll take a wild stab & hopefully I’m right. I’m guessing you’re talking about the Piccolo guitar in the Free plans located in the Resources tab? https://www.cigarboxnation.com/forum/topics/piccolo
If you read, on the first step, it says: build a classic 3 string CBG which uses G Modal tuning strings?https://www.cbgitty.com/guitar-instrument-parts/cigar-box-guitar-pa...
This project basically, cuts a 25” scale length fretless CBG in half to 12.5” scale length, but doesn’t change the gauge of the strings. That’s how it gets the Mandolin, Uke type tones? FYI the tension is a little tight on this one so don’t be surprised, but it works?
Am I right? Next time include where you got the idea or a link to it or a picture maybe some details like scale length? It’s hard when there’s not much to go on? Cheers :)