You've certainly given it good thought. It would be interesting to have students even consider what equability looks/feels like to themselves and others, perhaps with "field" experiences if inhabiting various online spaces (youtube comments, reddit, open discussion forums, etc)
Some quick responses...
They do not "give" you the plugins, but once you set up a wordpress site, you can easily install any plugin you like via the wordpress dashboard (tools you do not have access to in Wordpress.com). There are some steps/and things to with a site to set this up (covered in my Feed Wordpress 101 post)
A bit of clarification- the limit of 10 means that is the limit of who can be on camera/microphone. Giving 25 people access to talk can get really messy and hard to manage well (people will have cats/kids in the background making noise, people will not wear headphones and generate feedback). This is not the limit of people who can view a hangout.
What I would do / have done is create a schedule where students sign up to be co-hosts, or heck, put them in charge to lead the discussion, so they take turns speaking, so you can have a more manageable conversation with say 3-6 at a time; the others can watch, submit questions. Some can be google jockeys or twitter monitors. The beauty is everything is almost instantly archived.
I'd do some thinking how necessary this is- you already have a big list of sites/tools for them to use, and I try to minimize this as much as possible. I can see perhaps a reason for the subject nature for private discussions. When you set up a Reclaim account, besides wordpress, there are about 10 different web -based forums type software you can create. I cannot vouch for them because I never use forums.
Suggest they be consistent, if possible in username, avatar, that they consider using a pseudonym, and use the same email when they use services. I suggest also creating a gravatar account which is used by default in many blog comment systems.
I frankly think you have a bit too many tools on the plate. As much as I prefer twitter, I might make a choice/coin toss between twitter and G+, the latter might be better since they will need accounts already to use hangouts.
I also like to space out when I introduce new sites if they are more specific to the content or when they will be needing to use them (like you probably can defer having them create accounts on Vialogues, Genius until you assign them??)