I have just finished a Fleet Alert and the Tholians were the attacking side, never seen them in that before, they were awsome fun and a challenge in a way.
At one point Mirror Ships turned up? now that was a mix i wasnt expecting.
Best of all was that HUGE awsome looking Tholian Flag Ship, it was a really cool beast!
Was real Fun!
Have faced them quite a few times now, but have to agree - they're the better of the 'current' Fleet Alert enemies, especially with the Mirror Ship sneak attacks - played an instance involving the Tholians at the weekend; a mirror Defiant put in an appearance; really enjoyed chasing that one down in my Wells class, blasting it with Chroniton torpedoes to hamper it's maneoverability.
Not looking forward to the Undine putting in an appearance though - only a matter of time, but I'd imagine that a lot of time will be spent in respawn oblivion in instances involving them.
The mirror ships are a special power the Tholians have. Watch for the big glowing rifts - they bring mirror ships through the same way they captured the Defiant in TOS. You usually end up having to fight them yourself, but they're technically hostile to the Tholians, too.
I've yet to see Borg or Undine at Tier 3 in fleet alerts (seen all starbase incursion enemies but Undine) -- perhaps both are Tier 4?
Most likely tier 4, as you say.
Haven't seen them yet either, but as more and more T4 bases are popping up and, in a PuG instance, the mission supposedly picks a base at random from one of the players present, it's only a matter of time before they do put in an appearance.