As someone relatively new to Reflex, where should I start? What videos, communities, players, or other sources should I watch, look out for, contact, become a part of, etc?

phgp.tv is a good resource. For seeing servers/players in-game from browser: reflex.f3allready.com.

Also IRC: #reflex @ irc.quakenet.org

Is it possible to learn movement with a relatively low sensitivity? If I', able to do a 180 in either direction without lifting my mouse off my mousepad is that enough? What are some good maps for just messing around to get comfortable with moving?

What is relatively low? I got ~32.5cm/360 (which is relatively low I think?) and I move alright. As for maps if you want to gradually start learning the basics try Parkour Station by memphis, for map specific jumps reflexjumps and then there's b0_reflex.

The Reflex forums - http://www.reflexfps.net/forums/

As for aim, any recommendations on becoming better in this regard?

Work on movement and learn the items. Getting more items than your opponent normally is better than out-aiming him. Of course if you can do both then you are doing pretty good. There are players with "bad" aim that constantly win because they can control the items. The best way to control the items is to get there before he does. Thus learn movement.

GLHF!

Yeah it is possible to learn movement with a low sens. But in my opinion why would you? It's not a generic realistic fps, you actually CAN use both ends of the spectrum and be a top player. But if you start off with the low end I don't think you'll ever find movement and consequently learning it as fun as you would with a high sens if for no other reason than low sens being more tiring. Don't make the mistake of copying your sensitivity from one game to another, especially if it's very different than this one, which chances are it is.