I wanna to know if I can use a contract that is already deployed by others in Rinkeby?
It depends on what do you mean by "use a contract".
Rinkeby is a testnet so using contracts deployed by a third party have no warranty they are functional or that they will remain functional in the future.
Many projects deploy testing version and they are available for anyone to use.
If you want to reuse the source code as long as you follow the license it should be fine. If you have doubts it will be better to ask a lawyer.
Yes. Anyone can call any contract on the chain if they know the address of the contract. Now, when the contract receives your call, it may disallow the call because your calling address (i.e., your wallet's address, your address, your sender address) is not allowed to make the call -- but it will have received the call and decided not to process it. To make such external calls you need to know the "ABI" of the contract (i.e., which functions it implements). You can see this on Etherscan.