You can without any concerns plug a stereo cable into an unbalanced mono jack; as long as you only send mono signals (i.e. L and R channels equal) it will work just fine, except for the usually -3 dB pan law and possible short-cicuiting of the ring connector; the latter is however unlikely and not harmful for line connections. (It can be harmful for e.g. headphones connections, which you shouldn't be using.)
You can also plug a stereo cable into a balanced mono jack, but there's a caveat here: if you send mono signals, the L and R channels will exactly be cancelled by the common mode rejection, so you should pan anything you send hard-left. (If that's not a problem, you should also pan hard-left for
stereo->unbal.mono, that also solves the two small problems I mentioned there).
Both balanced to unbalanced mono vice versa usually works without problems, except obviously you won't get noise surpression in either case.
Plugging a mono cable into a stereo jack will not work well. If it's unbalanced mono, the result will come out only on the left channel; if it's balanced it will come out L/R in opposite phase. Neither is usually acceptable, that's a major reason why hardly any professional device has stereo inputs.