Essential Specs for new network

Hi,

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.

Luke