Icinga, which is a Nagios fork, allows Oracle DB's. Not entirely sure if Nagios does too. Advantage of Icinga, for me, is the fact that their new web interface (still in beta though!) simply kicks ass. It's extremely customizable, and has everything we need.
That being said, it's a monitoring system, and not a management system.
I'm not sure what you mean with "integration API (configuration, data access)". Do you want to set certain parameters through the NMS ? In that case, Nagios, Icinga and Zenoss won't be for you.
As far as scalability goes... it depends how you build your configs. If you put it all in 1 file, which is the default install and many unexperienced people seem to do (I'm unexperienced with this software myself and still setting it up for us, but I do realize this will cause problems in the future) you will end up shooting yourself in the foot. Split them up and use the inheritance functions!