OK, thanks for the screen shots -
1) Your settings all look good.
2) If the darker of the two images you posted is the original, and if the brighter image you posted is the result of applying color correction, the resultant grain apparent in the corrected image is just the consequence of having to bring the level of the dark picture up to acceptable levels. The original is severely under exposed, and bringing the level up in post is just revealing the sensor noise present in the image that is normally masked by correct exposure.
There is no solution to this within FCP, if you absolutely need the shot you can consider third party software such as Neat Noise Reduction:
https://www.neatvideo.com
(embarrassingly bad web site, terrific product) but, in addition to the expense of buying the plug in, it is very time intensive to render. It can do wonders in certain situations.
3) Convert your BlackMagic footage to the same settings as your sequence, then import them into FCP for editing. Should work seamlessly.
MtD