Rules of using Minions (In DnD and Movies):
1. Do not rely on your minions
2. Minions are expandable
3. Never send only minions, mix in some capable guys for fun and action.
Now some rules specifically for DnD:
I would advice you, to level the minions up to Level 6. Otherwise they would be way to easy to hit, and every Burst will wipe out many of them. Same with normal Zombie Brutes. Never use enemies of lower Levels, except Artillery and Soldiers. (These can be up to two levels below the players) Your normal Zombies are there for the damage, the minions are there for assisting the grab and the attack.
To summarize it (all enemies are Level 6):
First Wave: 4 Zombie Rotters, 1 Zombie
Second Wave (3 Rounds later): 8 Zombie Rotters, 2 Zombies
Third Wave (3 Rounds later): 12 Zombie Rotters, 2 Zombies, 1 Elite Zombie (The savage Berserker Template is always fun

)
For a total of 24 Zombie Rotters, 4 Zombies and 1 Elite Zombie. Do not tell them which one is a real Zombie and which one is a Rotter. The first hit will tell them. Let the Rotters asupport the Zombies by assisting (gives +2 to hit) and flanking (another +2). The Zombie can then grab and try to isolate the target. What then happens you already know from the morst Zombie movies
Encounter Level: 13 * 250 = 3250 XP = a bit harder then a level 11 encounter.
That may sound hard, but there are a few points that make the combat easier.
1. The enemies appear in waves. Most minions will be down before the next wave. The Zombies will at least be bloodied.
2. Play them like zombies = dumb as hell
3. You may give the players a round to prepare, go in position etc. as they see the hoard close in.
4. Clerical anti-undead Stuff will work