It seems like the 7th fleet title was made for the Interdictor, yet conventional wisdom would tell you this isn't legal as the "Interdictor" is not a Star Destroyer. Perhaps this isn't true. We need to do a little digging into the history of this ship to clarify this.
In the old canon of Star Wars, the "Interdictor" was known as the "Immobilizer 418 Cruiser," which was built by Sienar Fleet Systems rather than Kuat (the Star Destroyer manufacturer,) and was its own weapons platform separate from any line of "Star Destroyers."
However, the new "Disney canon" version of the Interdictor as seen in Star Wars Rebels is not that same ship. As well, the version of the Interdictor that we have in Armada is the Rebels Interdictor, due to the visible differences and being paired with Admiral Konstantine, a Rebels character who was commanding an Interdictor on the show.
The show Rebels does not explicitly state what kind of ship this is, so we can't use dialogue for clarification. However, allegedly, a Disney endorsed Star Wars fact book, released October 3, 2017 as part of the Journey to Star Wars: The Last Jedi book series, called Star Wars: Absolutely Everything You Need to Know, Updated and Expanded , refers to the ship used in Rebels as an "Interdictor-class Star Destroyer ." If this is true, then the Interdictor can equip the 7th Fleet Star Destroyer title.
To summarize, we know:
1: The Interdictor in Armada is the Rebels version of the Interdictor, rather than the old canon "Immobilizer 418."
2: The Interdictor in Rebels played a prominent role in the 7th Fleet
3: The Interdictor in Rebels has recently been classified as an "Interdictor-Class Star Destroyer " according to a Disney endorsed fact book made as promotional material for The Last Jedi .
With all that being said, I believe the Interdictor in Armada should be able to use this title, from a legalistic standpoint. From a gameplay standpoint, the interdictor is crying for upgrades to make it more relevant, and any little thing will help, especially with extra damage mitigation tech that synergizes so well with it.