I'm referring to the specific system of DnD 3.5 and to Zombies being slowed with the spell Slow.
Zombies already have "Partial Action" in that they only have a standard action per round. I understand the maluses to AC, Reflexes and so on would be counted, but I was wondering if there are official rules that perhaps say that, once slowed, zombies act every other round or something similar.
(interesting house rules are welcome as well, if there is no official answer)
The same thing that happens to everything else, they're slowed. Unfortunately it doesn't stack.
Slow notes that:
Multiple slow effects don’t stack.
Zombies, specifically have
Single Actions Only (Ex) Zombies have poor reflexes and can perform only a single move action or attack action each round.
Contrast with slow:
A slowed creature can take only a single move action or standard action each turn, but not both.
Therefore, the slow spell will do exactly what it says, limiting the zombie to a single move action and providing a penalty to attack rolls and saves. But, because the slow spell explicitly says it doesn't stack, and the text is the same, there's very little room to suggest that it does stack with a slow-equivalent effect.
Single Actions Only (Ex) and the effect of Slow spell (or effect) are not penalties of the same type. It would be ridiculous to, for example, say that the subject of a charm person spell can't be slowed because 'slow spells don't stack', it is only slow effects which do not stack, slow effects being a specific kind of effect (sort of like size altering effects) which generally affect the speed or number of actions of a creature. Single Actions Only (Ex) is not a slow effect because in order to be a slow effect it would have to be labeled as such. You could house-rule it as a slow effect, but that would be a bit silly and also unnecessary.
The effects DO stack, it's just that it doesn't matter that they stack because they both limit the zombie identically: while they both limit the zombie to a single move or standard action each round, neither of them actually reduce the number of actions by a fixed amount and so they coexist but do not further reduce the actions. As a note, if you rule that they do NOT stack, you should be aware that the misc penalties from the slow spell are part of the slow effect. Generally for non-stacking effects only the most extreme effect applies, which in this case would mean that the slow spell takes effect and the zombie is freed from its extraordinary ability temporarily. If you had a slow effect that just decreased speed by 30 ft with a minimum of 0 (so the zombie couldn't move), you would need to remember that effects only interact when they affect the same statistic, so the speed decrease and the action limitation, being different, would still effectively "stack" insomuchas both effects would be active and fully functional.
You may want to see: Stacking for rolls, here
While that text specifically talks about die modifier stacking, the same concepts are implicitly generalized to effect stacking throughout the rules (e.g. "multiple magical effects that increase size don't stack"). They are not explicitly generalized because moderating effect stacking basically comes down to the GM determining what effects do and do not count as the same thing, since no rolls are being altered.
