I tried with 3 medium sized nodes, and ran into installation failures. Then switched to a 4 node cluster (1 small for Ambari alone, and 3 large for HDP). I still ran into issues, but was able to figure out from logs that the Postgres port 5432 was not open. Once, I fixed that, I was able to start all the failing services like Hive, Oozie, namenode, jobtracker, etc. Ambari is a much improved installation program over HMC. Do not have to install from scratch if you run into issues with a particular service. Kudos to Hortonworks for making this much needed change.
I have all the services running fine, except the metrics only show for one of the 3 hosts. The ganglia and nagios services seem to be running fine, so not quite sure why it is not reporting metrics for 2 of the 3 hosts I have configured.