Hello
Not sure here if you mean 10mm needles or US size 10.
I adapt patterns and every sweater is a new learning experience.
My first thought is that whilst your stitch and row gauge might be fine halved for a square or blanket to acheive the same size, there comes an added level of calculation when it comes to any kind of shaping required in a piece of clothing.
If, for example, there is shaping to bind off or decrease by 1 stitch each end every 2 rows, you can’t just bind off half a stitch every row, a decision has to be made about how to shape and slope. With short rows in a shawl neck collar I would expect this to add a substantial amount of trial an error an experimentation to get the short row shaping smooth, correct looking, the size you want and so on working within the parameters of a pattern for a different gauge.
I’m not saying it isn’t possible, only highlighting what comes to my mind so you can ask yourself if you are prepared for this. If you are, great, go for it. If it’s starting to sound lie a bit too much design and maths calculation for you right now then maybe more research is needed before going ahead.
If it was me I’d hunt down a shawl collar pattern in the gauge or needle size I want to work in and then adapt that, maybe even reading through the pattern to see if I could add a button band and shawl collar from one pattern (in the right gauge) to another pattern (which is not the right gauge but I know how to size it with my chosen gauge). Or I’d try a swatch of the short rows involved in the pattern with the chunky yarn and larger needles and just see what problems and solutions arise from the swatch.
I’m definitely not well practised with short rows…but the little I know makes me feel the shawl collar is
the tricky part here, not the bulk of the cardigan. If the hardest part to alter is the most important feature, well, it would give me second thoughts.