I realize that if I have a wallet with 1 BTC and I receive .5 BTC, have 2 confirmations on it, spend .5 BTC and then it turns out that transaction wasn't legit I will just end up spending .5 of my original 1 BTC... But what if if I have a wallet with 0 BTC and I receive .5 BTC - how soon before I can transfer/spend that .5 BTC?
This is not how Bitcoin work. Bitcoin client select random inputs, with preference of oldest. And you can send even unconfirmed inputs