There's no issue with PG having their check-in in T2. It only takes a couple of minutes to get from T1 to T2 (either via "automated people mover" or walking). I find it quicker to walk. T2 is just shops/restaurants and a number of check-in desks for smaller airlines (such as Air Asia). All gates and security/immmigration are in T1. I often pop over to T2 to grab a coffee in Starbucks as the branch there is often quieter than the landside branch in T1. So if you can't use the transfers desks in T1, you'll clear arrivals in T1, walk straight over to T2 (there's a underground walkway from Arrivals....it's probably a 200m walk) to check-in and then back to T1 to clear immigration for your Samui flight. Slightly annoying but HKG is so efficient that it's a lot less painless than it sounds.

Not sure how strict PG are on hand baggage as I always check a bag in (bags usually arrived quickly at both HKG and Samui so never had the need to take hand luggage only to avoid lengthy carousel waits) and have very little hand luggage. But if your BA bags look obviously big/overweight, you may have an issue and be forced to check them in.

PG are one of my favourite airlines and the flights to/from Samui are the best. Cabin crew very efficient and friendly. Food is usually good. Not sure if you know but PG own Samui airport so they pretty much run the show there. It's quite a unique airport. There's a free lounge for all international PG passengers at Samui - it's basically the international gate (in a contiki style hut) but there's free (non-alcoholic drinks) and food, plus free wifi.