Clicking the active operation gives a choice of 3 difficulties: Easy, Medium, Hard.
When viewing the three difficulties, the current number of players queued for each is displayed.
Maximum slotted ship rank has no bearing on the operation you enter into, except that the Hardest difficulty will be limited to Rank 13 and above.
Alternatively:
Only the easiest difficulty is available for public queue, anything higher requires manual grouping.
Let’s be honest, nobody wins current Tier 5 difficulty with randoms, it’s always wasted time.
It’s a huge pain having to de-slot your ships in order to play a lower difficulty, just let any ship play the lower tier operations.