Well, to be honest for a moment, I would only recommend Yoko if you have either the body or the the tits to pull it off. If you have both or maybe even just a nice pair of legs, then what are you waiting for? People look upon cosplayers with respect, and they know how to look and not touch, even though they wish they could., so what is there to feel uncomfortable about? Are you insecure about your body or something? When you cosplay, you need to put your effort into becoming the character, rather treating it as a mere costume - Yoko isn't insecure about her body at all, it's hot where she comes from, and all the Gurren Lagann fans know this.
All you have to do is wear a bikini top, and shorts, and you could always make her jacket if you wanted to cover up more.
Nia is doable, you just need to have a yellow-silver colored wig and either be shorter or have a slender frame.
I just didn't want anybody cosplaying from School Days since it really is quite a shitty anime and Makoto fags don't deserve to have their bitches turned into 3D, you know? Everybody likes Yoko.