I bought the rx433n and it says to connect a 30-35cm antenna to the ANT pin. Does this mean that I should connect a 30cm monopole (wire)?
Yes, a piece of wire will do the job.
And ~30cm is correct? Some friends was arguing that the receiver is probably matched to 50Ohm and should therefore have 1/4 antenna - about 17cm.
17 or 34 cm to be correct, however, testing reveals that it is not that critical.