So....how does that work in the cryptocurrency world? If I buy $20 USD worth of Bitcoins...and then want to use all of those purchased Bitcoins to buy Ripple....will I end up with the same amount of Ripple (Ie. $20 USD worth of Ripple)?
Generally yes. You will most likely have $20 in Ripple as the market price will always balance. However there might be some short-term price variations, which will allow for margin trading.
Are cryptocurrencies not traded against each other in terms of valuation?
They are.
I feel like this answer contradicts itself. If cryptocurrencies are traded against each other, it doesn't make sense that buying $20 of Bitcoins and then converting that into Ripple will amount to a net equal amount.