Ratkin shooting is very nice, weapon teams are a very good units to cover your rear lines (just remember that as war engines they cannot move at the double, so deploy them with caution).
You can build very different lists and still have a perfectly competitve army,
KoW is not won during the army list selection. You will see some units recommended as must have, but very often it's more than the recommended unit is easy to use, but not always the optimal tool.
for exemple the Demonspawn (Verminlord) is a beast, and if you add the wings options, it's a true monster that can fight dragons and can do any job, even support nearby ratkins with inspiring and rallying(2), fire at long range with a decent lightning bolt(5) while you are getting in position, and finally use is 13 attacks in melee. (the number is not a coincidence
)
Will the wings, it's a very powerful and mobile unit that is easy to use, and many early ratkin armies included one, but the problem is that with the wings he cost as much as a hero on dragon (normal as he is in the same power level as one), and like a dragon, it's not always easy to use it at his best, as if you send him too far alone, you won't get to benefit from his inspiring (well, he can still inspire himself at least) and rallying rules.
If you are always in melee, lightning bolt won't see much use. And if you simply keep close to your other units, then maybe you might have done without the wings. And finally if you send him to kill weak units behing your ennemy lines, then you are probably overkilling them and using a 300+ point monster to hunt units and war engines worth less than 100 point each.
Make no error, the Demonspawn *is* a very good unit, it's just that a beginner might comes to rely on it too much, and due to the point cost be at a disadvantage with the rest of his army, giving even more the impression that the Demonspawn is required, and resulting ina vicious circle of the player relying on it more and more and not learning how to better use the rest of the army.
Now, you sometimes see one, but many players are trying to do without it, finding that you don't *need* him to win.
That said, my advice would be to first try to make an army as balanced as you can, able to compete in all aspects of the game, and learn from there what play style fit you the best before you over invest in any one kind of unit or tactic.
For slaves, they are nice and cheap, and many other armies would like to have them, but it's also true that many ratkin players already take many warriors or shock troops hordes, and as the slaves are irregular and don't provide rallying and don't give you slots for your heros/war engines/monsters, it might sometimes bee hard to use them as hordes too as it would make your army too unwieldy.
That's why armies with already a lot of other infantry hordes will tend to prefer using slave regiment as cheap chaff to cover their flank or serve as a screen.
If however you take less of the other infantry hordes or if you have a battle plan that combine those other hordes with slave hordes, like for exemple deploying in two waves with a screen of a slave horde for each warrior or shock troop horde hidden behin, then slave hordes or legions are perfectly viables.
Note : remember than you can move across your own units when not charging, provided that you have enough move to end on the other side. This mean that if your slave screen is heavily damaged but not dead yet, you might move at the double with the fresh unit behint to put it in front.
The damaged slaves might then serve as a new reserve force for later, maybe surviving the game and not giving any point, or they might hold objective while the fresh unit do the heavy fighting.
This can also work the other way of course, I once had a brute (rat ogre) horde in front of my battle line that took heavy damage from a early war engine hit, and they would probably have died at the first charge against them, instead I move slaves that were in reserve behing them to the front, and the brutes where now in cover, they were still exposed to another lucky artillery hit (indirect war engine ignore cover), but normal shooting now had a cover and the brute survived until the end of the game thanks to that manoeuver.
