There is no prohibition by Cal South that disallows players also registered with US Club Soccer from participating on a team in any Cal South program. The ECNL rules have a maximum roster size of 30 players. The CRL rules state the maximum roster size is 22, with game day team rosters being 18. So, what happens is the team will put all 30 girls on the US Club ECNL team and 22 girls of those girls will also be registered on the Cal South CRL team, with 8 reserved for primarily ECNL play, but also subject to club pass (5 of them on any given play date). In the event of a play date conflict, the CRL is pretty good about allowing a team to move that date to a weeknight (assuming the other team consents).
Now, if the ECNL branded team in the CRL play in doesn't make it, then no problem they just play ECNL. If the team does make it then they have some game day juggling to do on conflict play dates, but with 30 girls on a team, they can easily form two teams of 15.