Props to you for doing so much research. They look absolutely fine. Judging from your research I'm sure you already know this, but if you are also looking at Dirndl for the girls in your group, make sure to opt for ones that in the very least go up to and slightly cover the knee. In my experience, if a girl is really short, it's more acceptable to have slightly shorter skirts (as in ending just above the knee).
As other people have stated, it's not 'mandatory' to wear Tracht. In fact Tracht being so common during Wiesn is a more recent development (last decades). However, IMO if done right (and from what I've seen you are doing it 'right') it shows an appreciation of Bavarian culture and can actually be fun for everyone involved [I know, this is a bit of a controversial statement, because there are people who see non-Bavarians (including other Germans that are not from Bavaria / The south) wearing Tracht as offensive. The way they see it people are using their traditional wear to play dress up.] Also, most people will be wearing Tracht and you just might feel a bit out of place if you don't. It's not a bad thing, and people truly do not mind, but contrary to what others are saying it's not necessarily true that you will not stick out. In other words and without the double negative: you stick out, which might make you uncomfortable / care, but no one else does.