A simultaneously blooming crab will cross pollinate any apple. And no, it doesn't affect the fruit at all. Since no apples are really completely self fertile, virtually all are cross pollinated with another apple variety.....crabs are just another apple variety :-)

OK, cool. Another question: proximity... How close does another apple tree need to be in order to fertilize? There's a crab three doors down from me. Will that work?

Yup!! As long as you have bees present at flowering time, they will find both trees easily and make the trek between them.

Just keep an eye on the tree down the street. Our neighbor cut down his plum which was our pollinator. We didn't have a clue until one year we had no plums.