There hasn't been any "new" teams added to the squad in a long time, so adding Russel for that team would be a great addition. As far as I know, Sainz was in talks with them, no idea what happens now that he is in Williams, but if he can keep the deal then he may go there, so a George plus Sainz duo could be all a new team could hope for, and I bet that they won't be that great on their first year, who could be, it's a tough thing to build a great car right away.
However, if they have George and Sainz, those are racers that can tell them what is wrong and because of that I am sure that we are going to end up seeing them get their car better with time, it shouldn't really be a problem at all, they should be getting better results and get the car better with ease, thanks to that experience.
Sainz is already under contract with Williams for their long-term development. So there is no need to discuss him getting into Cadillac. As for Russell himself, we still cannot be sure whether he stays with Mercedes or not, because the rumours of Max entering Mercedes are still so hot, but I see that Russell will stay with Mercedes, so the chances of him entering Cadillac are still small.
The most plausible names for Cadillac are Mick Schumacher, Bottas, or Perez. These three drivers are rumoured to be being monitored by Cadillac to become their drivers for the 2026 season. Although it is still uncertain, these three drivers entering Cadillac makes more sense than Russell (or any other name).