can i use low fat milk instead of 1/2 and 1/2. i know it won't taste as rich but will it still thicken?

Hardlikearmour is right. The cornstarch will help it thicken, but what makes the pudding special is the richness from the half-and-half...I'd be interested to see what your experience is if you try it.

It should still thicken, but you're gonna lose a lot of the yumminess.