Mostly for bandwidth and connection stability reasons.. if the plex server is only serving your own home then likely not much to worry about but if you are serving plex content for ppl outside your home it can be troublesome if your wifi starts acting up and they don't know why they suddenly start buffering.
Nothing about plex itself is limited by wifi it's just a potentially a high bandwidth / throughput service and typically you wouldn't want that on wifi.
I think the premise was the plex server will be running off of wifi and they didn't think they would have wifi drivers available to them during install so they wanted to install hard wired.
No how wise it is to run a plex server off of wifi is another discussion all together but maybe for a different time.
Yes you can do that, make sure you are on the same CPU infrastructure (ie, don't try to install linux on an SSD from an intel laptop if you're going to be running it on an arm based processor or something).
are you doing the entire *arr stack as well? There are some cool tools to connect to plex as well allowing for things like requesting of shows that then integrate with he *arr stack.