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That scale length range is in mandola territory, so a mandola (which is tuned CGDA, a fifth below a mandolin), may be the way to go. You could also make an instrument of that length for other tunings, higher or lower or tuned in 4ths rather than 5ths or whatever - you just need to do some figuring with string gauges and tensions. I've designed an instrument that I call an "alto guitar" or "mando guitar" that has a scale length just over 18" and is tuned GCEA, like the top 4 strings of a guitar capoed at the 5th fret (it has double courses, tuned in unisons, like a mando). I'm planning to make a new one with 5 courses, to be tuned either EADGB (like the bottom 5 strings of a guitar, but an octave higher) or EADF#B, which would be like the top 5 strings of a guitar capoed at the 7th fret. If you look at the top 4 strings of that last tuning (ADF#B), you can see that it's the same as my original alto guitar tuning, but a whole step higher. I will use the same scale length, but lighten up the string gauges of those four courses to achieve about the same string tensions - actually, I'm going to aim for slightly lower string tensions, because the original alto guitar feels a little stiff to play (it's fine, really - it doesn't feel as stiff as a mandolin, but I want to make an instrument that feels a little looser and easier to play). Then I'll figure out what to use on the new bottom course.
If you develop your concept around an existing instrument with a similar scale length and tuning to what you're dreaming up, such as a mandola, or, in my case, a guitar capoed at the 5th fret, it gives you a starting place for figuring out string gauges and tensions. For the alto guitar, I just conceptually took a 24.9" scale length guitar and put my nut at the 5th fret. That's how I derived my scale length and knew, without having to do a lot of calculation, that standard guitar strings tuned at GCEA would be at a good tension. I used my 24.9" template to cut my fret slots.
Another way of looking at this is to observe all the different places on a guitar that you can play a given note. In each different place, you're playing a different string at a different fret - i.e. a different "scale length" and different string gauge - but the tension is always roughly the same. This illustrates how widely you can vary the scale length for any given note (or set of notes, i.e. a tuning for a whole instrument) by varying the string gauge(s). It also gives you a lot of possible string gauges and scale lengths to refer to for a wide range of notes, if you want to design an instrument with tensions similar to a guitar.
If there's anything I'm not making clear, please let me know.
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