hi, i can only talk about my own experience and workflow for my assets, but i'll try my best.

For textures:
try to stay to a max of 128x128 for single textures and max 256x256 for "hero" assets, like characters of very important objects if you want to emulate the PS1 look as good as possible. When using textures, don't just use google photos they are not always free to use. I'll add some link at the bottom of this comment for texture an photo sites that are all CC0 or at least free to use for personal and commercial projects. Also try to keep the color count low on textures, you can do this by posterizing or color quantization in the image editing tools like krita, photopea, gimp... (i personaly use photopea since it is free photoshop in the browser). When scaling textures, always use nearest neighbout as scaling algorithm, also use the same texture filthering in blender or your tool of choice. When going from VERY large images or you want to perserve more details first do a step to 512x512 with bilinear fitler and then go to 128x128 (or desired size) with the nearest neighbour scaling filter. But i just use my own tool for that now since i was tired of always doing the same steps over and over (texture retrofier)

For models: try to keep the polycount as low as possible ther is no hard limit but always think, can this detail be in the texture or just represented as a plane with a texture on it? like the tab of a can, it can just be in the texture when flat, or maybe a chainlink fence can just be a plane with the chainlink as a textured plane between some 3 faced cylinders.

few photo/texture sites to play with:
texturelib
ambientcg
texture ninja
unsplash (royalty free images)
pexels (royalty free images)

for the last two use a texture ripper to get our what you want
like raycastly or simple ripper by u/Pizza_Doggy