Your best option is 5x mages paired with 5x discs or 8x mages paired with 2x disc. I'd consider 5x warrior paired with 5x paladins but warriors got 0 mobility as main problem.

8x mages can devour 50 people in 3 gcd as there is no cap on aoe in classic, it is extremely best setup apart locks tbh in classic only limited by mana. I'd go gnome for 5% more int as well. Can also go for blastwave setup as well. Warlocks are good but can't burst large groups due to no aoe. As lock pally setup on the lv 30+ it's really easy to level due to hellfire setup being non interrupted from pally and lock talents.

Since horde has only 1 dispeller shodun't beissue but do know there is cap of 16 dots in classic and 8 locks can can only do agony plus corruption and your cap is full.

Overall, locks are best for smaller group i think and mages are better for groups of 6+ and above.