Can you mix and match Public Asset Library assets with Trimesh?

Yes, most of the assets from the Public Assets library are compatible with trimesh worlds. You can distinguish between the trimesh assets and sub-Ds, because after dragging the object to the scene, the property panel and the hierarchy will have an icon like this one for the trimesh assets:

They have removed the asset library from the desktop editor and moved all the assets to the asset panel by the console. All the assets in that folder are trimesh assets. But if you're working in headset than follow what PigeonNo12 said.