I'll be the first one to say not to cut them that short at the outset. Carbon is a different animal, and it likes to be long. I'd say get the dozen full length....or at least 30" and then bareshaft tune with probably 3 arrows before doing anything else. Use something like a piece of a plastic bag around the inserts to hold them in the shaft without glue while you go through the tuning process going up in point weight till you find what flies right.
You don't mention what your insert weight will be, but plan on needing more than 125 grains upfront. This will help you gain a higher FOC and better flight.
Good luck and have fun.