you do want to keep as large of a cable as you can, esp if you are using an mppt controller. losing the extra volt is not a big deal on a standard charge controller as they dump the extra anyway, but the mppt can use it. and running 10 gauge for 18 amps looks okay from the voltage drop end but dont do it.
as for the connections, you could use a butt connector to step down to 10 gauge, or a block as mentioned. i personally would very carefully trim a few conductors to make it fit as some one said