I believe nucleus will have their own wallet and your tokens will be there. The ICO's im in , have their own wallets and I see my coins there.
When you want to transfer them to your ETH wallet for safety purposes, then you can transfer them to the wallet (whether it's MEW or Metamask).
Metamask keeps updating functionality but you don't add MEW wallet to metamask, you import your ETH private keys to Metamask or vice versa. Since you're exposing your private keys anytime you transfer , this is one reason why I don't think you should be using two unless you're utilizing MEWs additional wallet functionality or Metamasks additional dapps functionality.
Like to hear
@Walt opinion on this but that's what I've been doing but there's several ways to skin a cat.