We all know Dragons are never really ones for fighting together, and that there won't be too many mages, as they are pretty rare, but what about difficulty with these, against the standard array of party, if they were so alone?
Basically, 1 person is never enough to kill a party. A party can be up to 5-8 levels lower and still defeat it (if they are over level 5 or so). It is because, while one is targeted (even by spells with a blast radius) there is limited party damage. Unless the single monster can cause enough damage to many targets, it is a hopeless battle as the PC's get more actions and outwit the creature.
Dragons are a good example of this, and many times people say "Dragons are easy!!!". They are possible to beat, but would they ever be alone? I doubt it. Many creatures would be its slaves, or creatures it allows to keep people out, while it lets them inhabit its (possibly) large lair. If the party came up against a Dragon, along with serveral squads of ready elite bugbears, they might have a harder time at even trying to attack the Dragon.
But performance is a key for many things. If there is 30 creatures attacking, no AI will keep the lag down. I would recommand at least +1 creature over expected players, maybe of higher abilites. Of course, my pretty bad 256MB, 1GHz machine probably will never coap with many things going on-screen at once - as my GeForce2 is pretty much minimum spec!
At higher levels, the creatures would need to be well equipped with the right spells and weapons or items as well, else they will be toast. Monsters with +2 swords at level 20 won't stand a chance against a mage's array of spells, or the massive AC of a fighter.
Single does have the advantage of little mod lag, but it is much more balanced to have equal amounts of actions being performed by the creatures as the PC's, unless balance is not the key.
Also, remember to place creatures (if possible) in ways they may want to battle in - PCs do this all the time. Mages at the back, and tough fighters at the front. Archers are good at the back, and clerics/healers in the middle. Adding missile weapons will really make them good at this style, as then mages won't need to melee attack, while there are still fighters there, and all of the creatures will be able to shoot like the PC's do, before melee errupts.
A party of monsters is almost always a good way to put life into a dungeon of standard monsters. Different classes, especially at least the 4 standard "Fighter, Rogue, Mage, Cleric" variety, rather then just fighters or just clerics. For persistant server worlds, this is especially as important.