Hi Hannah, I'll try to answer some of your questions, I've never made anything nappy related myself though, I'm too lazy (and useless at sewing) so I buy it all ready made!
Making wipes - Fleece doesn't tear I don't think but terry would so you'd be best to sew them. I find wipes one side fleece, the other terry best. I'm sure someone has step by step how to do these easily - something about sewing 3 sides then turning inside out to sew the last I think?? Sorry I'm really no help!
Boosting - terry would be fine, I think, esp if Erin isn't a heavy wetter, but hemp would be slimmer and I think I remember reading you didn't want it too bulky? Hemp can be slower to soak up the wee though - if you're using 3 I would use one terry at the top and two hemp underneath.
Sizing - (you're talking about minki pockets?) I've never used them myself but I would try the large for Erin I think, esp as you'll be boosting it up for night.
I hope that ramble is of some help, sorry if I've confused matters even more!