Hi, bit late to this one, sorry!
I use a medium Slinki Minki on my very petite DD, and I too only use it at night as it would be ridiculously bulky on her unfortunately for daytime. Poor thing can hardly waddle in it! But I just stuff it with a terry square, sometimes plus a microfibre cloth too, and it's fine at night. We've never had poo leakage, and only a couple of times has wee leaked (always when I've rushed and not fitted the nappy quite right, I think).
georgia & freya (17 months)