Welcome to Star Army.
1) Yes, you can play a Thaxarian. They're pretty rare but not restricted. I'd suggest sticking with a human-type character for starters though.
2) Power armor is what it sounds like--armor. It's a mechanized, armored exoskeletal suit that moves under its own power.
3) You could use a sword, and you could have a custom one if you wanted.
I can tell already that you'll probably have to watch your grammar in posts; so, when you get ready to submit an RP message, you might want to run the post through MS Word and proofread them just to be safe.