Most obviously, High Admiral is the Navy equivalent of High General,

and it would seem absurd to revert to "General" rank above the "Marshals": therefore, if Marshals go above High Generals, Fleet Admirals have to go above High Admirals.

Also, the rank of "High Admiral" is normally associated with a Sector Fleet, but it would seem absurd if there were no intermediary ranks between the thousand-odd High Admirals and the dozen Grand Admirals. Note that Pellaeon - commanding the Empire's eight remaining sector fleets, plus a strategic fleet of perhaps a hundred ImpStars - outranks a High General in the Hand of Thrawn duology... now partially this is by virtue of him holding the post of Supreme Commander, but again, we're back to the old problem... we need at least one rank above High Admiral for flag officers whose commands require them to be able to instantly outrank High Admirals... and Fleet Admiral fills the gap very nicely....

We also have to throw in the rank of "Grand General" (held by Malcor Brashin from Force Commander and perhaps by Carvin in XWRS)... another "General" rank, obviously packing some serious seniority, but again, it feels wrong above the "Marshal" ranks, and it's not a white-uniform rank in spite of being a "Grand" rank...

Now the "Wing Commander" and "not a mere captain" variants may both derive from a system (WEG?! XvT?! George?!) which assumed a single rank-structure rising from Captain to Commander to Colonel to General to Admiral, but that's thankfully contradicted by the consistent usage of reasonably 'normative' navy and army ranks elsewhere... and Davip's promotion in TUF confirms what's always seemed to be implied by people like Kratas and Pter Thanas, namely that "Commander" is used in place of "Brigadier" by 'army-rank' officers outside of the 'real' army (Starfighter Command, gunnery, etc.)...

[Then we're faced with "Captain" Soontir Fel of the Pride of the Senate, outranking "Commander" Rosk and "Commander" Toniv, but later becoming a Major in Starfighter Command... but that can be rationalized by assuming that Fel, Rosk and Toniv, were all naval "Lieutenant-Commanders", but Fel was the 'captain' of the ship...]

I am sure there are some cases where you could name a ship with a title beacuse with just Ackbar, and not a whole name it applies that his whole family is dead!! Was his first name ever mention in any book or comic?

The closest I recall to a Wing Commander is the Colonel i/c Fighters under Pellaeon in the HoT duology. His rank was equivalent to Group Captain, so I figured he was either Pellaeon's Senior Staff Officer (Fighers), or was the active OiC of the fighetrs immediately attached to Pellaeon, whether he commanded just Chimaera, or a squadron.

Could Fel have acquired dual ranks within different services? I don't know how common this is nowadays, but under Nazism, SS officers often held dual ranks. Either as SS officer and high level Police Officer, or as Waffen SS general and the equivalent in the regular army. Also, some foreign dignitaries are awarded ranks in foreign nations' armed forces. If memory serves...Kaiser Wilhelm II held a fairly high nominal commission in the British army prior to WWI.