I am concerned about the losses when converting flac files from 24bit to 16bit. Does it reduce quality or add noise or what?
Technically it does reduce the quality, yes. It should not add any perceivable noise. Will you notice the difference? Try it and see. I don't expect you will. 16bit is CD quality, ie pretty good considering how most people listen to music these days.
Some reading if you want to understand more:
A nice image from that page describing it:
As long as you dither, you'll be fine; it can overcome the dynamic range limitations of if you had recorded 16-bit.
Scroll down this page for more info on bit depth/dithering (I suggest reading the whole page):
If the FLAC files are not meant to be used for further sound design, and are recorded at reasonable levels, you don't lose anything important. The dynamic range of 16-bit sound is very high - roughly 100 dB (how dither affects this makes the answer not quite as straightforward as you'd thing it is).
If you are truly concerned about the audiblity of bit depth on files like that, the best way to reasuure yourself is to test for yourself. Get a free ABX comparator like FooBar, put in a file at 16 bit versus one at 24 bit, and listen for a difference. Here's another blind test already set up that you could try: http://ethanwiner.com/BitsTest.html
I have pretty good hearing, checked with blind tests here: http://www.audiocheck.net/blindtests_index.php
I can assure you that unless you're planning to further process the sound or the sound in the file is very very quiet, you won't lose anything important. If the sound is very quiet, normalize it first and then you can reduce the bot depth without losing anything important.