Is Benvolio's description of Tybalt as the "fiery" Tybalt an apt description of his action, behavior and language?
I would say that this is most definitely an apt description of Tybalt. It seems like Tybalt is always angry, spoiling for a fight. This is how he is pretty much every time we see him in the play.
When we first see him in Act I, Scene 1, he wants to fight Benvolio when Benvolio tries to get the servants to stop fighting.
When Tybalt sees Romeo at the party, he gets really angry. Lord Capulet really has to slap him down to keep him from going and making trouble with Romeo right away.
And of course, Tybalt picks the fight that leads to Mercutio's death and to Tybalt's own death.
So Tybalt is a very hot-headed person.
If you take a look at Tybalt's own words in Act I alone you see him as fiery, or fiesty . . . always looking for that fight.
In scene i: "What, drawn, and talk of peace? I hate the word As I hate hell, all Montagues, and thee. Have at thee coward!"
In scene v, Tybalt refers to Romeo as a slave, and then he uses the word "villian" 3 times.
Finally, in that same scene, Tybalt says, "I will withdraw; but this intrusion shall, Now seeming sweet, convert to bitt'rest gall."
If Tybalt isn't fighting, he is threatening to be fighting in the future.