Hi there!
It kind of evades my why your consultant has chosen not to install the other components as well 
But yes, you will be totally fine by installing all the components on the vCenter server VM. You could separate SSO server and Inventory Service on different servers VMware vSphere 5.1, but that is more suitable for larger environments - you would have to have additional Windows Server licenses for each of those separate services.
Also there is a nice blog post about inventory service http://nickmarshall.com.au/blog/2013/6/24/the-vcenter-inventory-service
Hope this answers your question 
Edit: According to what a.p. said it is more than possible that they are installed already - check your services if vCenter Web Client is started.