I like the sgs/projectdarkstar or now known as reddwarf server for a back end game server. However I am looking for a good front end, preferably 2D. I was thinking of something such as flixel. I know there is an AS3 library available for reddwarf, so I am hoping someone already has connected up flixel or a similar engine? Or if you have a recommendation for another easy client side engine that will connect to reddwarf?
Flixel and your client engine don't 'connect' to each other. They are both libraries YOU have to integrate into your flash.
Think of flixel more like a box of pencil crayons, and your client library like a phone cord. Alone they don't do anything. It's how you use them that makes your game what it it.
Also judging by the rough bounds of this question I might recommend you scale down your first project of this nature. Server/Client interactions are a hefty challenge on their own, without any graphics ever being involved. But before you ever try that, make a full game, start to finish with flixel, then add new technologies. If you can't be patient enough to take the time to do that, you will have serious trouble ever finishing this project.