The Interdictor Cruiser is super old EU, but super fun stuff too... Were they at Endor, and other planets? Tough to say. Scarrif seemed to be set up for defense, not attack (IE: blockade of a planet) which is what IC's do. The big half circles are the gravity well generators. They didn't work in EU like they do on the toon where they sort of "snap back" a ship trying to leave. They simply caused gravity wells artificially so a ship couldn't hyper out... IE: like a planet is in its way.
They are, however, a far weaker ship. They're significantly smaller than a Star Destroyer, more in line with a Victory-Class Destroyer, which was also an EU vessel pulled from concept art for ISD's from the OT. They were supposedly a late Clone Wars Era vessel, like a lot of things in the EU that basically Lucas decided to ignore rather than intelligently pull from and integrate if even in small doses (like Y-Wings, Z-95s and other stuff).
At Endor like I mentioned before, the battle was intended to be MUCH larger, but effects such as they were at the time, left a lot of things as simple specks of a background overlay that got more and more complex shots layered atop it. If you look close though you can see ships that appear a GREAT distance from the battle taking place... It's kind of neat. Same can be said of the Rebel Fleet at the end of ESB actually. Tons of ships, just some are way far off it seems and hard to make out the specifics. But it's totally plausible Interdictors were at Endor but held a distance away... which was kind of what Konstantine's vessel was doing till he decided to break rank like a ninny. They could take on a smaller ship... And while that "carrier" is big, if it's anything like a ship that existed in EU that basically served the same purpose, it was a weaker vessel and relied mostly on what came out of its hangar than what it had as its own weaponry. So the Interdictor going after it for a kill is kind of silly but possible.
Interdictors could be relatively easily taken out by Y-Wings and B-Wings though, in games anyway which had pretty good balance to them.
As far as numbers there seemed to be plenty in the gaming world but two at a battle would've been ample to bog down anyone fleeing the scene too. They're kind of neat... I believe they too may have been derived from concept art for Destroyers. Not positive on that though, but for some reason I'm thinking they are/were.
I'd like to see some Victory-Class Destroyers and some of the Venator-Class peppered into the show, personally. Something to imply they're there and in service in some capacity. Just like in Rogue One a Juggernaught was in use, I like to think these other larger ships are still serving a purpose in the Empire. Hell, if you went by EU the Empire has Corellian Cruisers of various types, and the Nebulon B and its brethren are actually Imperial-born ships too, all part of pickets and larger fleets.
