Essential Specs for new network


I am currently in the process of developing a new website and I am unsure of which platform to use.

The essential specs for the new network are as follows:
1. It need to be 'own labelled'
2. It must have the ability to link to an external site keeping it framed within the social site, for example, using <iframe>. Some third party sites require login credentials I would like to be ‘passed thru’ from the members profile so the member only needs to log in once.
3. It will require one network with many sub-networks. Each sub-network should be private and cannot be viewed by other sub-networks. However, I would like members from any sub-network to view other sub-networks' members being able to invite them to become friends. There should also be a cross sub-network forum.
All other cross sub-network function should be hidden / private. The main site activity feed should be set to show only items from the members’ peers in their network only.

Firstly, I was wondering whether this is achievable with elgg and how?
Secondly, if not, do you know of another platform that does offer this functionality?

Any help will be greatly appreciated.