If I have Dengar on swoop (speed 12) and and the klat captain (granting speed 8), is the speed 8 considered a penalty because it is slower than his printed speed or is it still a bonus because it is faster than the standard speed 6, and more importantly how far can he move on his turn?

I would think that he could still use speed 12, because both should be bonuses, but just thought I would check on this.

Great question;
take my brainstorming with the appropriate amount of salt hehehe

does the klat captain CE apply to followers, and does Dengar on Swoop have a CE? (ie, is Dengar on Swoop a follower)
Does Klat Captain's CE have a range, or is it board-wide?
Or, is Dengar using some 'transport'/passenger ability to move the Klat Captain?

If Dengar on Swoop is a follower, and does not have special abilities which ignore CE's (droid, savage, independent outfit etc)
and,
Klat Captain has a CE which applies to 'followers', allies (not a specific title etc), then yes,
The speed of 8 overrides Dengar on Swoop's printed speed of 12. Dengar can't ignore that CE, unless he had savage, loner, independent outfit/character somehow etc...
this is based on _NickName_ and Sithborg's previous reflections that, you can't choose to ignore CEs from your own squad or voluntarily fail saves etc...

So, Dengar on Swoop would ordinarily move 12 and attack (which I think is move 12 strafe attack), or move 24 and not attack or use any ability which replaces attacks.
If Dengar can't ignore the CE, and is a follower,
his speed would change to Speed 8: move 8 and attack, or 16 and not attack

However, if the Klat Captain has a CE which makes followers within a range have Speed 8,
then, Dengar is only reduced to that slower speed when within the range of that effect (or, within range of a MouseDroid, or if Mas Amedda is on the squad, etc).
This can be abrogated if the character with the CE is within range of an enemy with Disruptive, when the character with the CE is defeated etc...
Confused yet? So am I, it can get quite technical hehe.

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If Dengar on Swoop is using "Transport" or a similar ability to move an ally,
that would function similar to past rulings on Rebel Troop Cart, so,
the ally is 'removed from play' while 'stowed' on the character with Transport,
and you'd use Dengar on Swoop's Speed 12 for the purpose of moving etc.
Once you complete the move, you designate an adjacent square to the vehicle, and that's where the ally is moved to.

EDIT: disregard my approach - as it is in error hehehe

Worse is using the glossary and missing the key phrase. (done that several times)