How to evaluate quality of signal?
Indeed, in practical systems there is usually a cut-off SNR above which communication is possible with a given amount of reliability. This is usually determined according to a design goal, expressed in terms of a bit error rate (BER) or a packet error rate (PER). Typically, the BER should not exceed $10^{-4}$ or $10^{-5}$.
According to your signal parameters (modulation and coding), sometimes it is possible to derive the BER as a function of the SNR. This depends on the complexity of your transmitter-receiver architecture and the channel model you use.
So:


Yes.


Now, regarding the RSS, I would say that it is not a standard metric. Some equipments give RSS metrics, according to an unknown criteria. Sometimes, it is just a table where a given range of received power is indexed. Hence:


It depends. In general, we don't use the RSS to directly determine design parameters of a demodulator (in this case, a threshold). But if you know exactly what it means, you may derive a relation SNR-RSS. But since RSS measures the total received signal power--including noise--I suppose you will have to estimate the noise power also.