Is it legal for USA residents to use exchanges that are outside USA? Like cryptopia? I know we can use coinbase and Bittrex.. any legal advice here ?
Yes, there's no law against that. There's a separate issue of whether any given exchange prohibits US residents -- Bitfinex and Wex do, for example. Cryptopia allows them, though. In general, just check the terms before you sign up and fund an account.
Technically, I don't think it's unlawful to conceal your location to trade on Bitfinex or Wex (with a VPN) either. But it would be violating their terms of service and they could probably get away with confiscating your funds if they caught you.
I'd imagine the smaller exchanges like Cryptopia don't care yet.
At the rate this market is evolving, it probably won't be long now.