Green tulip poplar will check and twist reliably and it's placid grain can react in ways you will not predict. There is a tendency towards spiral grain. I have used tp but prefer it for secondary wood free of heart but sawing straight lumber can be a challenge in longer lengths.

Anchor seal the end grain and oil the rest to keep the drying rate slow.

I generally dislike the hammer beam configuration and would disqualify tp as major framing timbers in a hammer beam frame. Your hammer beam drawing gives the impression of an English hammer beam without some of the essential elements. I believe you'll get a better frame if you use a king or queen post truss design. On other condition is to not trust tension joints such as the principal rafter foot without additional relish.