You may only need 15 pounds of the sand, but 20 pound bags are generally the smallest available.
You should not try to mix Kaudern and Pajama Cardinals in a 10 gallon tank.
This is not at all the "normal" way to set up a marine tank. A normal marine system is set up with live rock, aragonite sand, and a protein skimmer. The costs add up very quickly. However, there is nothing wrong with gaining a little experience on a simple tank.
Remember when you set up your first freshwater tank. You probably kept a pair of Guppies or maybe just a Betta bowl. This is kind of like that. You get used to some simple concepts, get your hands wet, and learn a little. In the mean time you read, read, read, read everything you can on these threads. You also save for your "real" tank. Then, we convert the 10 gallon into a quarantine.