OP, are you flying TPE-HKG-PVG on CX? Or TPE-HKG on CI (China Air) and then HKG-PVG on CA (Air China)? But either way it doesn't matter:

From what I understand, you must present BOTH a *G card and a *A ticket in order to be admitted into the Thai lounge (or any other *G lounge for that matter). CA is not yet in *A. And your RCC card will get you into the Thai lounge only if you hold a TG ticket. I'm afraid you cannot use the Thai lounge in this instance.

Your RCC card will get you into the RCC of course, without needing a UA ticket. I flew BR HKG-TPE a few times and have always used the RCC without problems.