I'm looking for the smallest XLR mic I can mount to my FS100 for use with a gimbal. It's strictly scratch audio, nothing more than for ambient or sync purposes. Cardioid pattern preferred. My Google search has only come up with short shotgun or pencil condenser mics. Any lighter/smaller suggestions would be most welcome.

Barring an XLR mic, could I use a lav mic with 1/8" to XLR adapter? I know that most of these are omni pattern, but that is secondary to size/weight considerations.

I've read, many folks like the small, low cost Audio Technica AT875R. Length 6.89" (175.01 mm) Diameter 0.83" (21.08 mm) Weight 2.8 oz (79 g) $169 USD street. The $400+ Ambiant ATM216 TinyMike is smaller/lighter but has a mini-plug (see below).
"Barring an XLR mic, could I use a lav mic with 1/8" to XLR adapter?"
- Yes and no. most lavs need 'Plug-in power' (aka, bias current) so an XLR Phantom Power adapter/transformer would be needed, That adds at least $100 to the price of the mic. I wouldn't recommend a camera mounted lavaliere for even scratch track purposes.

Here is an XLR microphone that is about the size of the XLR connector.

Naiant Studio - X-O omnidirectional condenser microphone (http://naiant.com/studio-electronics-products/microphones/x-o-omnidirectional-condenser-microphone/)

Richard, you're a lifesaver. Checking those Naiant Studio mics out now. Do you have any experience with them?

I don't have any experience with that model. But I do have some of their previous models. I found the company to be honest and reliable and producing decent products.