1. As an engineered system, Exadata is available in a set of pre-configured rack increments; the Exadata datasheet at http://www.oracle.com/technetwork/server-storage/engineered-systems/exadata/exadata-dbmachine-x3-2-ds-1855384.pdf has the full details about sizes. Additionally, it's possible to add more storage to an existing Exadata environment through a storage expansion rack (datasheet at http://www.oracle.com/technetwork/database/exadata/exadata-storage-exp-rack-ds-v1-425092.pdf)
2. The replication options on Exadata storage servers are comparable to other systems running ASM on a raw, direct-attached disk. And yes, there are replication options other than Oracle Data Guard, including GoldenGate, Streams, Materialized views, etc. But not replication tools that require additional filesystems or storage controllers.
3. Although you can technically run general-purpose software on Exadata compute nodes, they can't take advantage of the Exadata storage servers, so there isn't normally a huge benefit to doing so. If you're looking for a general-purpose application platform in Oracle's engineered system family, consider Exalogic or SPARC SuperCluster.
Hope this helps!