If I cast on with larger needles and get the correct # stitches per inch but the rows come up short, what can I do or is it ok to start like that?
Add more rows to the pattern where length matters.
It’s very difficult to match both the stitch AND row gauge given in a pattern … everyone knits differently, so you’re probably not going to be able to match the pattern creator’s gauge exactly.
From what I understand it is more important to get the same number of stitches per inch as the pattern specifies … if you are getting this correct then I would not worry too much if the row guage is off slightly. You can always knit more/fewer rows if needed to make sure your item ends up the correct length. 
Don’t worry about the row gauge so much, most patterns give the length in a measurement, not rows, so you can just work to match that.