Need some rechargeable batteries for my X300UA, and my M600U Scout.

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16340/RCR123 is rechargeable , CR123 is not.

I can't help you with rechargeable in that size, I buy only primaries (CR123) in that size because they are cheap and most single 123 sized lights these days are quite efficient.

Battery Junction is a good place to start though is you want some

I have had very good results with the Nitecore branded 123’s. I use them in a Camera and they last months longer than anything else I have tried.

Look into AW 16340s. No one has a better reputation than they do. With that said they are just rebranded Sanyo, Sony, Panasonic, ect cells (same as everyone else) but they are known for using high binned cells and stand behind their products. You'll pay a bit more but they are as quality as it gets. Beyond AW look into most any decently sized companies cells. Just avoid Ebay.

So doing a bit of googling turned up a couple of forum threads apparently. There can be issues with voltage and rechargeable batteries? I think my M600U and X300U are both 6 volt lights? All I am seeing is 3.2 and 3.7 volt RCR123s. So if i used 2 of those I would kill my light right? What is safe to use? Am I wrong somewhere?

Depends on the light and its circuitry / LED and what they can handle.

CR123A = 3.0V nominal, normally ~900-1400maH capacity and ~2C output ( 2xC, C=capacity, so 900-1400maH x 2 = 1800-2800ma, or 1.8-2.4A)

RCR123 or 16340 = 4.2V max, 3.6-3.7V nominal, 3.2V rest, 550-750maH capacity and 1.5-2C output (so 550-750ma, .55-.75A).  Also, due to small size, you want protection circuitry to prevent too high of draw (prevent overheating so they don't ignite), and low voltage shutoff so they can be recharged - this circuitry makes them about 1mn longer than CR123As.

So, I don't know if the SF lights you have are RCR123/16340 compatible from a voltage POV, but I do know just based on capacity you'll get shorter runtime, and potentially less lumens based on output.

Are the ones Surefire sells (LFP123) crap?

The LFP123As are 3.2V 1.92Wh (so 600maH capacity), no clue their output.  But compare to their normal CR123As, which they claim to be 1550maH (probably closer to 1400), and you can see the runtime is less than half.