1. Not sure if i understand that right. I'm using GPO (Windows Active Directory) to push a cmd script to all computers in our LAN to launch Spark installer with a silent switch while PC is shutting down to install.
2. Depends on the firewall. On the server the 5222 tcp port has to be accessible to Spark. So Spark must be able to go to remote 5222 port (it will use some random outgoing port, so outgoing connections for Spark program should be allowed on a workstation, which should work ok with Windows firewall and many other firewalls as they usually allow outgoing connections). File transfer is a bit different as it works PC-to-PC, usually it should work. Though we had problems when using Windows Firewall together with Symantec endpoint protection's firewall. Problem is gone after we have disabled Windows firewall. Are you talking about the performance impact on the firewall? Not sure about that. Spark will send small packets every second or so to update its presence information. But no huge traffic usually. Not sure about the security part of the question.
3. Spark is only a desktop client. No mobile version. You can use some other jabber mobile client, but it will look differently.
4. No official requirement. But as it is based on Java, you should use some i3-i5 CPU at least and 2-4 GB of RAM. It will still feel sluggish at times. As it is a Java based app in theory it is OS independent, but that's just a pretty theory. It works on Windows. There are also installers for Linux, but some users are having problems. There are also problems with Mac OS X (latest). So the only system that Spark is working best on and is tested most on is Windows (XP through Win8).
5. If you mean that one client runs it on Linux and another on Windows, then yes, they will be able to connect/chat.
6. There is no requirement for AD. You can create local users on the server.