Will I be able to buy and sell cryptocurrency and keep a fiat balance on the exchange?

Yes.

The PlutusDEX is used for buying Bitcoin, Ethereum, and of course Pluton. On the PlutusDEX, your fiat balance is held until the purchase is completed, and then the balance is transferred to the user who provided the cryptocurrency.

The Plutus Tap & Pay app is for “selling” cryptocurrency because you receive a fiat balance after depositing Bitcoin, Ethereum or Pluton.