questions about an external recirc skimmer

[Saltysteele]

anyone have experience with external recirc skimmers? is height a factor in placement? for instance, are there any rules as far as how high not to go after the skimmer (like higher than the skimmer, higher than 3/4 of the way up the skimmer, not higher than the riser tube)? what is it that sets the water level in the skimmer? with my in-sump skimmer, i raise or lower the riser. i've seen the gate valve models and mods. if for some reason, i get a surge of water into the skimmer (either gravity fed or pump fed), what is going to keep it from overflowing?

really, i'm clueless when it comes to external skimmers (or recirc skimmers for that matter), so any help would be appreciated