Hello,
It is interesting project for us. We are experts in C/C++ and network programming, you can see some examples of our work at our portfolio more samples you can find at our site.
I see here that bidders ready to provide you with ready solution in 2,7,15 days....I'm sure that they didn't read your specs! In 2 days or 15 impossible to make stable and bugs free solution. At the start need to design the architecture of application, design protocol of communication (it can be XML based for example,etc), etc.etc.etc. From your specs we didn't find mention about the Server side- need to use a server or application will use broadcast massages, etc? also you didn't mention about how application will works with sub networks. I mean that one user will be in [login to view URL], and another will be in [login to view URL], or you want to chat only inside one subnet. About Firewalls, if firewall well installed and configured, he will cut packets from your application- you will need to make a Role at your firewall for your application. So, what I want to say if you have a realistic budget, and insist on high quality work, service and communication we will be glad to hear you.