As far as I know, USTA mixed is always a combo feature but the ratings cannot be more than 1.0 apart. Also, I believe most USTA mixed leagues went to a 6.0, 7.0, 8.0 level format so they would be compatiable with the mixed playoffs at sectional and national. But a doubles team at 7.0 can still consist of a 4.0 player and 3.0 player.
The other things you are referring to sound more like local play than USTA. Your local club sounds a lot like the ALTA format used in the Atlanta area. It is always interesting to me to discover how others are playing.