Copay Multi-Sig Wallet 2 of 3, possible issues Copay wallet was created by one of the co signers. Is it possible for this co signer to change the 2 of 3 rule without approval from other co signers? Or add another new co signer without existing co signer approvals?
If such changes were made, the address would change so it would not effect you. You give out addresses that have a specific set of signers and that cannot change. A different set of signers would be a different address, and if you do not know about this different set of signers, you would not have these other addresses to even give out.

So no, it is impossible for a co-signer to change the other signers nor is it possible to add more co-signers to a given wallet.

It is possible for a co-signer to create a new set of addresses with other co-signers added and then give out those addresses. But unless you were specifically told about this, you would not be aware that you could sign for those addresses.