First Ex-raid, take who?

I'd take your Houndoom, Scizor, Snorlax, both Dragonite and another of your choice :) (Gengar's got very low HP and takes more damage from Psychic attacks due to its poison typing).

With an ex raid, as long as you have more than 7 people average about level 28, you can bring whatever you have. I have seen one person with 4 high level accounts on 4 devices "solo" the ex raid before. Just bring your mons with the highest CP and you would be fine.

*Sarcasm* Smack down tyranitar and machamp
(those two are trash at Ex raids)

*seriously* take dark types, like a dark tyranitar if you can do raids for those, otherwise, houndoom and see if you can get that scyther into a scizor, gengar could be good, but its fragile

Others will cary you anyway if you make it. It's pretty obvious. Start off with Gengar, then Hundoom and Scizor are your only good counters. Fill up with your Dragonites and Flareon. Snorlax will not deal any damage.

The following 6 have the best DPS against Mewtwo out of your list:
Gengar
Houndoom
Scizor
Alakazam
OR Dragonite
Flareon

Gengar is super squishy though, so I would probably bring the Hu Dragonite as the sixth one instead. You shouldn't have any problems if there are a decent number of people at the raid.

At that level your best bet is join in a large group

My pick are scizor, houndoom, 2 dragonite, snorlax
And maybe Gengar, altough at that level it cannot use the Shadow Ball if Mewtwo has Confusion, for Psycho Cut probably can fire one SB

Ignore the comments that Gengar is squishy.

Use Gengar as your first pokemon - he will survive long enough to use at least one Shadow Ball (possibly two, if you power him up / Mewtwo has Psycho Cut + Hyperbeam or Focus Blast). He will get you an early lead in damage (you're competing with the other players for damage output to get extra balls) and could make the difference to getting you an extra ball.

After gengar, I'd go with Houndoom and Scizor, then Alakazam and the two Dragonites.

Surprised you don't have any Tyranitar from the community day...

Kind of depends on the moveset. If SB, avoid psychics, and Gengar/Zam will get creamed. If FB, Gengar, and to a lesser extent Zam, will wreak havoc for you. Dark Ttar is great (but you don't have any - work on that, especially after Larvitar CD) unless it has FB. Psychic is probably one of the easier movesets, and HB hits hard (especially if partly cloudy), but isn't too bad either. Scizor isn't a bad choice, but it's kind of soft.

Try to get a sense from the other players there what the moveset is, and adapt your team accordingly. And remember that raids are about doing max DPS while taking your lumps.