Thank you for the responses thus far.
I have got some rat swarms, 4 units of which so I should be good up to 2K, though I'll have to see when I start playing how big a role they playin my army.
I have looked into the various Proxy options, leading to large orders with Pendraken, Kallistra, and Splintered Light. However, from all that I've turned up online regarding the Pen/Kal Ratmen line they are out of proportion with GW's 10mm, being closer to 15mm, and one person I talked to suggested I'd be lucky to fit 3 or 4 on a Warmaster base.I like their ratmen sculpts enough to get them anyway, but I'd like to have an army that looked appropriate up against my friend's GW Undead and MM dark elves.
I've seen the range of ratmen MM currently have, and while they appear to be closer to the right scale, their Range at the moment is incredibly limited and the rats look a little "nice" for Skaven, almost like cuddly hamsters with spears. But I've got a small test order in to see.
I would really like to acquire as much possible a GW Skaven army though, their sculpts are just gorgeous. To that end, I had a bit of luck on ebay just this morning for a few things, but surprisingly the unit I would have thought the most common (clanrats) seem to be the hardest to find (I guess because everyone needs lots of them).