Can PGA be cascaded to amplify small signal?
Yes, certainly. The first amplifier will typically dominate the error and noise. If you have a PGA204 with gain of 1000 and the input offset is 50uV, then you could have 50mV of output offset from the first stage. Another gain of 300 would put the input offset beyond +/-15V rails. Same thing with noise. If your requirements allow you to AC couple the stages then you can get rid of the offset voltage issue, since each stage will only amplify its own offset. If you can band-limit the noise coming out of each stage then you can reduce the noise amplified by the next stage. 

One possible reason to cascade such amplifiers is to get better frequency response by keeping the gain relatively low in each stage. The input-referred error and noise of each stage will generally be somewhat worse at low gains, of course.