Hi, there. Welcome back to knitting!
Is gauge all that critical? On a cowl, I'd say close is close enough. If you work a swatch and like the fabric done with your yarn and whatever size needles you used, then figure out how many stitches you need to obtain the size you want and cast on that many. Seed stitch is simple enough that adjusting the number of stitches used should be pretty easy. My concern with going with smaller needles to make gauge is that the fabric you create would be too stiff. I just encountered this very problem making socks, I was past the heel turn when I realized it just wasn't working out the way I wanted it to and ripped it all out and started over with larger needles. HTH