The thing with sign and trades is that the player has to be interested too. If some other team wants to pay an unreasonable price the Warriors won't match, then Ezeli will just sign with that team, and leave the C's and Warriors negotiations out of it.
Sign and trades for RFA's make sense only if the contract is something the original team would consider paying but is uncertain about, either due to luxury tax or overall value reasons. Otherwise:
1) If the offer is more than the original team would ever consider, they aren't going to match, so no reason to trade.
2) if they think it's a great deal, they'll want a lot of compensation in return, or will just match.