Solidity – Why ERC-20 Interface Uses uint256 While Uniswap Router Uses uint

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}      

If you wish your token to be ERC20 compliant you have to have the required functions with the required signatures. That's the whole point of the standard.

However, that leaves you with two other options:

1) Add more functions and/or contract variables. You can add whatever functions you wish with whatever signatures and functionality you wish as long as the required ERC20 signatures also exist in the contract.

2) Do whatever you want inside the required ERC20 functions. ERC20 can be considered only as an interface - it doesn't tell you how to implement the functions, only enforces you to have certain functions in the contract. You can for example add some data with a custom function and in the required approve function use the data in some way.