Note that we recommend even powers of two for the number of data nodes (2, 4, 8, 16) and replicas (1, 2, 4). We also recommend that you *don't* run other processes on the data node hosts.
For your larger case, I think you'd get better results with 4 or 8 data nodes with lots of CPU and memory, a couple of less bulked-out machines for the SQL nodes, and a couple cheap low-end boxes for the management nodes (a 400 MHz CPU and 256 MB RAM is fine for an MGM node running on a no-GUI Linux machine).
Of course, there's nothing wrong with running ndb_mgmd's and mysqld's on the same machines but for production use, just make sure you've got at least two of each and that you don't have all of the management nodes or all of the SQL nodes running on the same box.
Jon Stephens
MySQL Documentation Team @ Oracle
MySQL Dev Zone
MySQL Server Documentation
Oracle