Yes for the moment you "can exchange" btc for doge , ltc and btcTestnet ; no at the moment it is not safe to store your coins on it because it is an alpha version
Actually storing coins in Mercury is already pretty safe, it's the trading part that isn't perfect yet. (However, the keys are not yet encrypted).
Can you add in the future version the possibility to see the private key for each address that I own in mercury? Maybe also the opportunity to import/exports the various address.
Thanks for the attention.