Getting your textures from cc0textures.com should be fine. Since they've been released into the public domain, they can be used for whatever purpose. The file format chosen shouldn't really matter as long as the resolution is consistent, i.e. a 2k JPG is just as good as a 2k PNG file. The only real difference is that PNG files support transparency.
With respect to image resolution, go with one that suits the game's look. The pixels of lower res textures will be more noticeable in 4k resolutions, so going with a high resolution texture is a good idea. Though it depends on the art of the game. Just because the models have 4k textures doesn't mean the player is going to notice these high resolution textures (though I'm sure the player would appreciate such detail). One of the drawbacks of high resolution textures is that they take up a lot more space, so a balance may have to be found between higher resolution, game size, and art quality.
The placement of the other materials should be fine: the normal material goes with the normal map, roughness material goes with the roughness map, and so on. This answer suggests putting the displacement material in the Depth attribute. As for triplanar mapping the only disadvantage I can see that the engine would do more blending on the model, and thus more processing.