You can never be too warm, I'd suggest a 3mm or 3/2 full suit. Lot's of folks use shorties in the tropics, but if you do long dives or repetitive dives you'll want the extra insulation. You might also email the liveaboard operator and ask for their recommendation.
Also, some folks, especially atheltic males; cyclists, runners, skaters, and dancers have trim issues due to their heavy legs. If you fall into that category, definately go for a full suit, since the neoprene on your legs will help offset negative bouyancy and help your trim.
As far as northern diving is concerned, I don't think this would be a worthwhile base for layering, but ask around locally and find out what folks are wearing.
I consider wetsuits as consumable, and suggest you shop around for the least expensive suit that meets your needs and fits well. Have a great trip, dF