It's perfectly ok to cross her legs in the Mei Tai if she tolerates it.

Some babies (including mine) just don't like having their legs crossed inside a MT, they prefer legs out. An adjustable-bottom MT allows you to adjust the width of the panel bottom to make it narrower for small babies and for FFO (front facing out).
The most comfy RS would be one made from double-layer silk, double-layer linen, or a woven wrap fabric (like Storchenwiege or Didymos). XL fit is not an issue with unpadded ring slings, you just get a longer length.
