The 512 MB/1 CPU system is too small. The 1GB/1 CPU can technically run Cassandra but it is too small. You should likely start with 2GB/2 CPU systems but expect to upgrade.
Select them in the same region for right now.
I went with Ubuntu 14 and the commands here will presume that. The commands were done from the root account. This will also presume that you are doing a multi-node cluster (3 is