See this thread for info about the scene in the Lower Mainland.
http://airsoftcanada.com/showthread.php?t=157768
To answer your questions;
Any recommendations on where to rent from? What field to play at?
Optix (user on this forum) runs the Airsoft Gear store and also does rental packages ($65 includes field fee, 1000 BB's, Goggles, mag, battery, and I believe a hot dog and drink). I believe they usually play at Ambush Paintball on the weekends. Also drop by his store on 8060 Anderson (right across the street from Richmond City Hall, kitty corner from Richmond Centre). It's easy to access off the skytrain. Also Trigger Airsoft is nearby as well I believe on No. 6 road between Cambie and Westminister.
Believe none of us have our own camo or any other "tactical" type clothing, will that be an issue? Can we play in relevantly dark/neutral clothing (assume long sleeve/long pants and good boots)?
Yes, you can totally do a "civilian contractor" outfit. BUT avoid black, navy blues are much better at blending in than black. Even pitch dark (or small amount of light from the moon) black stands out more than navy.
Any suggestions or tips for what to bring that wouldn't be typically be supplied with a rental?
See above, and check out airsoftgear.ca for the rental package details. As for what else to bring, water and lots of it. Also nice if you freeze some of it the night before and put it in a cooler so that it acts like a cold pack and you can drink it when it's melted and still pretty cool.