Leave the plant alone it will sort out how many fruits it will set. I read several studies made in China and most ruled out PT as a rootstock for Honey Pomelo.
There is only one way to find out if it's incompatible to FD.
In Southern Thailand they don't use any rootstock for pomelo. They use air-layering. It's vary easy to do it in summer. Then you will see which grows faster and fruits better. I put my bets on air-layered plant.
It's being raining here everyday heavily for the past two weeks. None of my citrus lost even a leaf. They are all doing great soaked in water. As long as the temperature is over 18 C they will do just fine.
Millet is right about the root problems, but I see them start with autumn cold rain, not now.
I dunno, it has been coldish here lately ,due to the rains and not much sun, at day the temps are around 20-25C ,at night they are around 10 c,quite cold for the season so far.
Ok I wont thin any more of its flower buds, will try to manually pollinate as much as I can , Ive heard citrus does not need hand pollination ,but I dont see many bees usually at my yard to do the work for me.
I really like the idea of layering, thought the plants is very small so I dunno ,on own roots it may bring out its vigor since I have a seedling which is one summer old and its already as tall as the grafted one,started to branch out nicely also....it does not show any stress whatsoever and it was in the same condition as the grafted one...
For the sake of safety I brought all my citrus under my polyethylene greenhouse ,away from rain.