Overview
I’m going to develop an app on Android and iOS. It will allow users to set up group ‘chat rooms’ and talk on chat rooms set up by other users. The service needs to be highly scalable, such that it could accommodate a massive increase in users overnight (we can only dream).

Chat requirements
The chat protocol used should be flexible: it should allow me to determine who can view/post on ‘chat rooms’ based on certain other factors, as determined by the first poster/creator of the particular ‘chat room’. It should also allow for users to simply install the app and begin using the service, after only providing a simple nickname (which could be changed later).

Chat protocol plans
Having looked around I think the XMPP protocol is the best candidate. In particular the Multi-user chat extension looks like what I’ll need. Would this be most suited to my requirements, or do you know another potential solution?

Cloud service
I have been deciding between Amazon Web Services, Google App Engine and Windows Azure. I’m coming to the conclusion that Azure will be best, as it is easier to manage than AWS (ease of scalability will be a key factor in the design), I think it may be less restricted than GAE, plus Azure will soon have toolkits to allow easy interfacing with both Android and iOS phones.

Is this the decision you would have made, or would you recommend/look into other cloud services?

General project philosophy
I have only recently started looking into this project’s feasibility, and am no expert on any of its aspects. So wherever possible I will leave the actual implementations to experts, i.e. choosing a higher-level cloud service, using a well-documented plugin of a, proven reliable, group chat protocol etc.

My background
I have some programming knowledge from a computer science degree. Main languages I’ve used have been Java and Python, but I don’t want this to affect design decisions for the project. The most appropriate languages for the task should be used, i.e. I don’t mind learning a lot of new skills (my current programming levels are relatively basic anyway).

Thank you
Thanks for reading, and any advice you have about any aspect would be greatly appreciated :-)