Is this possible for the players to change the core rules of the game?
No. No hacking either. This is blockchain my friend.
If someone hacks your wallet they would get your stuff though right?
What do you mean hack your wallet?
To get access to your account, you need to connect your ETH address with Metamask. What he means is, what will happen if someone has an access to your ETH address.
For example, I know your private keys. And if I am connecting it to Metamask and login in to the game,
I have an access to your stuff, like Eggs, Monster, and Others.