What are the rules on using both of these in the same rifle? Are these able to be fired interchangeably from the same barrel or is a different barrel required? Why would one choose a slug over a regular pellet and vice versa? Upsides/downsides?
You didn't mention what kind of rifle and you're asking in the springer gate soooo.....what kinda rifle...lol.
Springers don't have enough power to shoot slugs. The reasons for choosing slugs in a PCP is they are easier to cast in heavy weights for hunting with big bores, and they have a much higher ballistic coefficient for long range shooting with "mid bore" pcps.
If your barrel is choked there might be disappointing results with slugs. My Uragan compact is shortened and now without the choke. It shoots both HN and NSA 21gr slugs very accurately.