There are four specials:

Mouse: Restart – one episode of behind the scenes / interviews and an episode 11 preview
Mouse: The Predator – two episodes showing events from a different viewpoint; the first is more essential
Mouse: The Theatrical Cut – the first 17 episodes condensed into 2 hours in sort of chronological order, not very useful
Mouse: The Last – one episode of behind the scenes interviews and some extra ending footage from ~50:45 on

This makes the correct full Mouse viewing order:

Episodes 01 to 10
Mouse: Restart (**optional**, no new scenes)
Episodes 11 to 15
Mouse: The Predator Episodes 1 & 2 (or at least the first episode)
Episodes 16 to 20
Mouse: The Last (new scenes from ~50:45)

There's no reason to watch Mouse: The Theatrical Cut unless you just want to re-watch a bit of Mouse without comitting too much time to it.

Thank you!