I'm a newbie with some technical skills, but not a lot.
Should I be using MetaMask for added security?
What does MetaMask do for me, exactly?
Also, MetaMask seems to only work for Ethereum-based coins. Is there an extension like MetaMask available for the other coins?
Yup, metamask is better than MEW because it offer more layer of security to your wallet, it also works for erc20 tokens and there's tons of them on the market so it's a big plus. Other coin that doesn't use ethereum blockchain has different official wallet and I don't think there's one for that, even hardware wallet like nano ledger s doesn't support all token but only some famous one.