schon mal vs je vs jemals
I know they all mean "ever". But when do i use each one??
Schon/ schon (ein)mal vs jemals/je = with schon (einmal) you are more expecting a positive answer, je/jemals is more skeptical. Using the version with "mal" expects a time related answer, without it it sounds more like a yes/no question.
Those are all very soft rules with lots of grey area though.
"Je" can be used instead of pro/per to set something relation. If you had to set the number of cats in relation to the number of inhabitants of a town you could write something like.
In der Stadt gibt es 2 Katzen je Einwohner.
pro Einwohner, per Capita, pro Kopf would be other ways to express that.