I recorded room ambience at -50db and I'm not sure how loud it actually should be (in relation to other sounds). I realized that I started to use this to mask the noise of foley recordings, then I started noise reduction of foley so I can lower the ambience cause it was too noisy with everything combined. So, my question is: how should I record ambience, at what levels, and how is noise treated in those recordings?
Get your dlog levels right, and everything else gets mixed according to that. There's no specific level for ambience (or foley for that matter), it is simply mixed to sit naturally in relation to the dlog. It always frustrated me when people gave me that answer, but it's pretty much the only right answer for that question.
If you're struggling with noisy recordings, you probably need a new location to record in, or new preamps/cables/mics (depending on what sort of "noise" you're dealing with). A noisefloor of -50dB isn't all that low/great.
Noise reduction is generally not a very good solution for foley, as the sounds can tend toward the delicate side of things, and denoise algorithms generally don't "do" finesse terribly well. Unless a very light denoise pass will suffice, I would suggest just rerecording.
If noisy preamps are the issue, then grab a FEThead from Triton Audio. It's an inline mini preamp, inserted into the signal chain, that provides around 23dB of pristine gain. It's a great and easy way to significantly lower your noise floor.
If you're dealing with location noise, try recording during the night. Whenever I'm doing foley for projects, I just switch my schedule to working nights for a week or so... It's a lot easier/better to shutoff HVAC, eliminate traffic noise, people and pet noise, and have a solid 6+hrs to record - completely uninterrupted.