Hey Rango, I remember you from the Ike boards. Seems like a few of us ended up over here haha
Anyways, to answer your question I don't normally use fastballs between my uairs, though I know there are several different ways to go about utilizing the move. My method involves me using a rising uair, then as soon as I can, input another, immediately mash my double jump to hit with another rising uair, and then finish with one more uair at the apex of my second jump. Often times, you'll have to mix it up with a bair or nair if the opponent starts to DI away from the direction that you're going.
To be completely honest, it really just comes with time. When I was picking Mario up I was lucky to get 2 uairs in a string, now sometimes I get upwards of 6 (with the help of platforms of course). A lot goes into it though, whether it be reacting to DI, understanding the opponents fall speed/combo breakers- all that good stuff. May sound complicated but it's actually pretty easy, you'll catch on a lot faster than you think.
Best of luck