I've TPK a group of 6 level 6 players twice with only two of these guys.

Yes but considering a spike growth in their way and they fail their wisdom check they take up to 16d4 damage averaging at about 40 dmg per minotaur, then they don't quite pack a punch anymore.

I get a chuckle out of these guys, since they'd weigh roughly 20% of a live minotaur. 
Swinging a jumbo greataxe 1/3 its weight around would pull and swing its body around willy nilly.
Likewise its charge would have much less impact. 
In one game one charged a high level character, but did low damage and failed to knock back, 
then it got critted on and killed in one turn by the guy it charged. 
I narrated it that the guy dug his heels in, caught the horns, and the height difference and momentum carried it up off the ground, whereupon he chopped it in half at the spine, then decapitated it. 

Quote from WhatsGoingOn123 >>

Would this and a normal minotaur be a good boss fight for 2 level 9's and a level 8? If not, should I remove one of them or change the enemy?

 No, not even close. That's barely a mini-boss fight.
 A level 9 character will solo a minotaur pretty easily (usually).
With that in mind, 1 minotaur with 2 skelly versions could be appropriate.
It will still probably be easy, unless the party isn't fresh, or they don't have a barbarian (to tank) or paladin (to smite hard) or cleric (to turn undead).
Any martial with a maul or warhammer will make quick work of them too.
Think about it this way, a single fireball could drop almost half of this 2 monster "boss fight".
Rule of thumb, a boss monster should be much stronger than a single player, taking teamwork to down. 
Even in a 3v2, each monster should be a bit stronger than each character.

This thing is so cool!

El daño de 2d12 del hacha grande (greataxe) del esqueleto de minotauro es específico del statblock del monstruo y está diseñado para reflejar su fuerza, tamaño y naturaleza como una criatura grande y poderosa. Este aumento de daño no se debe a una propiedad mágica del hacha en sí, sino a las características del esqueleto de minotauro.

Si un personaje del grupo obtiene el hacha y la usa, debería tratarse como un greataxe normal, que hace 1d12 de daño, según las reglas de armas estándar de Dungeons & Dragons 5e. El daño de 2d12 se debe a que el esqueleto minotauro tiene una habilidad innata que le permite hacer más daño con el arma, probablemente debido a su enorme fuerza y habilidad para manejarla de manera más efectiva que un personaje promedio.

En resumen:

• El hacha en manos de un jugador hará 1d12 de daño (como un greataxe normal).
• El daño de 2d12 en el statblock del esqueleto refleja cómo el monstruo utiliza el arma, no una propiedad inherente del hacha.