Basically, the difference between bolt-on and set-in is the bolts vs. the glue.
If you look at e.g. Gibson, they have a little bit different contour on the neck heel. More like a key-hole shape, than the bolt-on rectangular.
There are rumours about some luthiers that make a dovetail! Never seen one of those, though, even on picture... And this would be a heavy duty, particular work, superjoint...so you might as well make a neckthru
Anyway, we all know of simple key-hole set necks that have been in business for decades. A simple rectangular joint is the same, perhaps with a little less glue area, so you might be a bit more cautious when selecting and applying the glue.
[edit] Oh, I forgot about the cello thing. It's well possible. What is the scale of a cello? I mean, a bunch of different scales are used for bass guitars, from some 28" to 38" at least. So it would probably not be any problems to make a cello-scale guitar with cello-gauge strings.