If the pool is empty (no liquidity), then you can fill it with whatever two tokens, with whatever ratio. To be precise, even Eth can't be used, but its token representation WETH needs to be used - although the Uniswap router takes care of this transformation for you, if needed.
Once a pool has liquidity (there are some amounts of two tokens), you can only add liquidity with those two same tokens. The ratio doesn't need to be the same, but it's best if it is the same.
If you remove 50% of liquidity, you'll get back 50% of tokenA and 50% of tokenB from the pool. Note that the ratio you get back is probably not the same as it was when you added liquidity, since the tokens have been traded in the pool. So if you initially enter 10 WETH and 10 tokens, and a month later remove liquidity, you may get 100 WETH and 5 tokens.