I have a client who is looking to have his ticketing system moved over to Salesforce using communities. The system that they currently use is web-based and their customers have logins for this particular system (On Demand) I have a few pretty easy questions for anyone who is familiar with communities, but I just want to confirm what I tell them is accurate.

1.) There is an option for Single Sign On for communities and I just wanted to make sure that Single Sign On applies in this scenario. My question is: Does Single Sign On have any limitations when it comes to using it? Would we be able to specify that the single sign on feature is related to the login for that particular system when using communities?

2.) Is there any limitations when it comes to communities with integrations compared to a production org? It doesn't seem like there would be any limitation with integrations and communities, I figure you can just reference specific profiles or the community organization id or something when creating any apex. Are my assumptions correct and if so, does anyone know where I can find some documentation on this?

I can't find the documentation that I am looking for that answers these questions directly.

We have an app that posts to Chatter when it performs a certain function and we're porting this to work in Communities. The one thing we're stuck on is how to set the FeedItem.NetworkScope for the Community from which the FeedItem is being posted. 

Is there a way to obtain the Community ID in Apex so that we can set it as the NetworkScope?