Depending on what era of zombie rules you are referring to, zombie rules in Necromunda are often regarded as a game breaking mechanic, as in they become overpowered from being able to wipe out characters through infection. That being said, I don't think there is much you can do with house rules to make them more unbalanced.
I've been toying with the following rules for zombie hoards (separate from those included in Scavvy gangs, have not looked into that at all):
If you are playing a scenario where the objective is to fend off waves of zombies, start the game with 6+2D6 zombies that enter the table either from their table edge or determined entrance points such as tunnel entrances, etc. At the beginning of every turn, add another D6 zombies to the table in the same fashion. Control the game with either an objective, allowing new zombies to arrive every turn, or as an attrition where after a decided turn, no more zombies arrive and the game ends when all zombies are killed.
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Hoard Zombie Profile:
M: 4 - WS: 2 - BS: N/A - S: 3 - T: 2 - W: 1 - I: 3 - A: 2
Special Rules:
Zombie Shuffle:
A hoard zombie moves towards its closest opponent every turn, and must charge an opponent if they are in range to do so. If a charge is attempted and they are out of range, they merely move 4" rather than stopping their charge completely.
You Must Sever the Brain!:
Zombies receive a 5+ Armour Save to represent that unless they are hit in the head, they will continue on. Note that this is an Armour Save instead of a Special Save as weapons with Save Modifiers are more effective at hitting a zombie hard enough to induce a head injury and put it down.
Relentless:
Zombies are immune to pinning.
Nasty Little Biters:
If a character is taken OOA from an attack by a hoard zombie and rolls an Infected Wound, Chest Wound, Leg Wound, Arm Wound, or Head Wound on the Serious Injury Table - including those rolled as results for the Multiple Injuries result - must roll a D6, and on a result of 5+ the result is changed to Dead and can not be re-rolled by using anything such as the Medic skill or Friendly Doc territory.
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Where the Attacks Value is 2, the additional attack die rule (+1 for not having a Basic/Special/Heavy Weapon) would not apply as zombies should never be packing heat, so they just get an automatic two attacks. Zombies are relatively uncoordinated (low WS), are fairly weak (low T), but still deliver a wallup from a successful hit (normal S). Keeping their movement normal rather than something like D3+1 allows for quicker movement phases granted the number of zombies.
I don't see any issues with adding bonuses like you have mentioned above, just adds more excitement
. If you have zombie figures of various scales (such as those for the Zombicide game), you could bestow bonuses on the larger ones and augment their Strength, Toughness, or Attacks values as you see fit.
