First, Dissidia is non-canon. None of Dissidia's characters or subplots are canon for their original games.

Second, Square chose Griever for some very solid reasons:

a) Square has been building the corresponding hero/villain relationships in every Dissidia pairing. Jecht was chosen as X's villain for his tie to Tidus. It was designed for better character development and more in-depth story. Choosing Griever gives Ultimecia a stronger tie to Squall, emphasizing their pairing in the game.

b) Square wanted EX forms to be recognizable to fans. Ultimecia doesn't have a huge set of abilities in FFVIII, but one of the ones we did see was her junctioning herself to Griever. Choosing Griever has a lot more instant recognition to fans than a separate monster for each character. It's something every fan of FFVIII recognizes and connects to Ultimecia, it makes sense to use as her EX form.

c) Creating new EX forms for each opponent Ulti fought would be time consuming and unfair to the other characters. Ulti-Griever was an already designed character. They had to adapt it a little, but that's much easier than starting character creation from scratch for 21 different forms.

d)Getting that many forms would also be unfair to the other characters, who don't get to totally change their appearance due to whomever they're fighting.