I see no problem in allowing them to fruit their second year if they've covered the area you want them to grow to.

Grapes normally aren't summer pruned. They are pruned in winter to remove most of the old wood. Leave 2-4 canes of last summers growth. On a vine that size I'd leave 3-4 clusters at most. It's really not that big yet.

I seem to recall reading somewhere that a bunch of grapes requires 10-12 mature leaves to feed it enough to ripen successfully.

So, count up your leaves, and divide by 12. That should be your upper same maximum number of bunches to leave on.