I want to be able to monitor all the devices including unix servers, windows, network and firewall devices under Splunk. Do we have any add ons that would enable us to monitor our devices instead of using SCOM or nagios.
It looks like you are looking for system health metrics which is the exact use case for ITSI:
As a start, you should try these apps:
Splunk app for Windows Infrastructure (it has pre-requisites): https://splunkbase.splunk.com/app/1680/
Splunk app for Unix and Linux (it also has pre-requisites): https://splunkbase.splunk.com/app/273/
Much of the rest will probably come in through syslog: http://www.georgestarcher.com/splunk-success-with-syslog/
In general, any device that logs in text / human readable format can be indexed and acted up on in Splunk.
See : https://docs.splunk.com and
Check out the community driven, and Splunk supported : https://splunkbase.splunk.com/apps/. You can browse all the of the community and Splunk supported add ons to match the devices you have.