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I currently have:
2 Warlord
2 Warlock Engineer
80 Clanrats
20 Stormvermin
2 Poisoned Wind Mortar
2 Warpfire Thrower
3 Rat Ogres
2 Packmaster
20 Plague Monks

From the IOB set and a job-lot I bought.
My next purchases are going to be a Screaming Bell then either a HPA or Battallion (followed by the one I don't get first).

Primarily, what's the ideal number of Clanrats to run? I've been working on the theory of squads of 40, but is this the magic-number?
Are Skavenslaves at all important, if so how many and do I just use clanrat models for them?

Also - general composition of the army, what gear should really be given to what etc.

First question. How many points are you planning to build your army for? That will do much to give us a framework to give specific advice.

Skaven slaves are usually in most builds as tarpits. Run them five wide to maintain Steadfast while you lock down an enemy unit for two or three turns. Your unit will die, but it only costs around 100 points if that so no big loss. Also, you can shoot into any combat that the slaves are in due to Life is Cheap. As for models, I used the skaven with no apparent armor on the back/shoulders and gave them hand weapons. This makes them look like lowlie slaves in rags with knives.

In larger armies, I run my Clanrats in blocks of 35-40 including any characters I put in there. Again, I run them five wide to maintain steadfast while letting my characters deal out some damage. They die so fast that running them in hordes really isn't worthwhile. The exception is if they're pushing a Bell.

I'd advise against the Skaven Battalion. Get more rats from the IoB set. You can find these on eBay for around £20 or so. Better value as you get basically everything the Battalion offers for less money (except the plague monks).

From what I heard 2000pts is standard? so I figure I'll start with that.

The first Battallion or two especially I'll either be stealing the plaguemonks for Dark Eldar converts or using them...

But for future items I'll consider that.

How useful are rat ogres / giant rat / plague monks / stormvermin units overall?

As always the rats start well with 2 IOB + 2 Battalions + Cannons + Blisters.

Slaves/Clanrats are kinda the same fig, ish. The special fig is metal and most folks don't use em.

I would skip the bell, it never earns it's points, the furnace can be good but you need a fairly strong unit of monks to make that work, and then it struggles to earn it's points most games. I would do a battalion then HPA.

"How useful are rat ogres / giant rat / plague monks / stormvermin units overall?" was the question

Not good enough to make most tourney lists is the answer, I like them all and they all do okay they just don't fit the mold and require support/build/too many points to just add in. I like Rat Ogres and play with them alot because I like them... no other reason. (they hit hard but are naked and require the packmaster to be effective) Giant rat has the same packmaster issue otherwise they are clanrats with spears for cheaper, Stormvermin fight like Clanrats but for double the points. Plague Monks can be effective but need to be used right as they are all offense.

The magic number for clanrats is 5 (wide) x 4 ( first rank +3 more for combat res/LD) + expected losses = 30 in my games with high slave counts, 40 in my low slave days. Big blocks of slaves make line of sight tougher along with protect from charges. Clanrats would be the bunkers for your chars most of the time and your tanking block for HPA/doomwheel/etc. Nothing is more fun as a Skaven player in my book than putting out a 20 x 10 deep block of slaves, it scares most folks to touch it, death frenzy/bless with filth makes it worse. So the unit touching it (assuming 5 wide) takes 3x7+2 ranks of poison attacks and if the unit explodes.... heh even better. Though I normally reform it when it gets under 75. The whole thing about Skaven is just how amazing it's cheap units are, get a friend and proxy stuff, its all a good time if you are not a need to win kinda guy.

The only real requirements for gear are a BSB, ward saves for your chars and a doomrocket on almost any lock, I throw him on my warp warlock I set up to be skitterleaped in the first turn to make him a real PITA.