As for your question: aircraft assignments between TPE and HKG are extremely volatile. They send whatever is available on the route. It's far too early to tell which plane you are going to get in DECEMBER. I fly this route several times a month and hardly ever get the plane advertised. Only the via-TPE flights to NRT, KIX, NGO, FUK, tend to have the same planes all year long.
Incidentially, your best FC bets for the evening are 451 which you mentioned and 401. CX401 is usually a 747 or 346, it's the so-called "banker flight", where first + business tend to be full of HSBC suits and consorts going home after a day's work in Taipei. For some reason these people insist on a first class cabin.
There are almost no FC services TPE-HKG in the morning, because the big birds come back from the US and Europe to HKG in the morning, then take off for their Asian day services. Your best bet for a morning 747 is 407 on a Tuesday.

F section of the lounge in TPE: you'll get in I suppose but believe me, you don't WANT to be there. It's the back room of the otherwise excellent lounge, a bit smelly, excactly the same food as in the big room (which has more seats, a tele, more newspapers, computers and is generally a lot friendly than the first class part.) And it defeats the purpose of an 'exclusive' section: it's always empty during the day, because there is enough room in the big room, and it's completely chock-a-block in the evening, when all the aforementioned banker dudes crowd it with their finance babble.