We only had one GS tournament where not all players get in. But at least all could play qualies as a few forgot to send and ALTs got in.
So it's assumed anybody tries USO and if you don't reach main draw there you can play Como. The key is: MD in singles and doubles. If you are in one of these main draws you may not play como. So the experience is that there aren't more than 40 players in GS first week challengers.
In second week Top 30 players are behind anybody else and a few players forget to commit until first deadline or even don't play. So usually it's always possible to play challenger qualies for unranked players when you commit before first deadline.
So commit for Como and USO until tomorrow 6pm CEST and you'll be able to try both.
E.g. have a look at Marburg challenger which was during Wimby first week: Tennis Tipping #26 - Marburg CH | Congrats to cah_ & frici/lucian_iasi!