Row gauge is much less important than stitch gauge. Once you've begun an item, you can always knit more rows to make it longer, or less to make it shorter, but you can change your stitches per inch.
Go with the needles that give you the 18 st per 4 inches, and when you get close to the armhole decreases you can determine if you need to knit more or fewer rows for the length you want.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry