First off, thank you for defining the power level you are looking for as that makes this a lot easier.
You are correct in needing more clanrats; two units of 40 are perfect battleline for 1000 points. The rat ogors unfortunately aren't going to work; you'd need more of them and a master moulder to make them perform. All the characters are good to go, but I would run just one deathmaster.
That leaves you with 300 points. A vermin lord could fit here; normally this would be a bit of an over-investment in characters but the vlords are extremely powerful. Another unit of infantry (take your pick) would also work, though note plague monks could be too strong and plague censers are a poor choice since they won't have the support needed. A grey seer on screaming bell would give you some more magic and battleshock immunity while leaving points for a small infantry unit to snag/camp objectives. Stormfiends are always a good choice, just don't take the poisoned wind weapon option on them.