Sorry, I only use a single remote for my blaster. I'm not familiar with how the hub responds to commands from multiple remotes (aside from the fact that I can control the hub with the iPhone as well as the remote, but that doesn't sound like your situation). Hopefully someone else can answer that for you.
To clarify on your second question, the harmony hub's IR ports are 2.5mm. Assuming that your 900 IR emitters are 2.5mm, you should be fine. However, if you need adapters to handle 2.5mm to 3.5mm step, I've got a post in this thread, back a few that details my adapter shopping list that should help.
Bottom line is that I've used the NextGen system for several years and thought that was the sh%t until I discovered the HSC system. Best control system I've ever encountered.
Edited by vestaviascott - 7/19/13 at 1:01pm