Russian Imperial Partigyle

I am planning a partigyle for Saturday and my big beer will be an imperial stout. Here is the recipe on Beersmith http://beersmithrecipes.com/viewrecipe/313327?&doid=5297ae622dfe6 . I will be mashing at 150 to get more fermentable sugars for the big beer. I may raise that temp for the small beer. It will be a 1/3 2/3 split, with estimated OG of 1.1 for the first five gallons in a batch sparge. The next ten gallons will be after another mash with a cap depending on what my small beer will be. Because of the size of my mash tun, it will be a continuous sparge. Estimated OG for the small beer is 1.05. I'm thinking about a dry stout for the small beer, but would like something a bit more exciting. What would you do for the small beer, and what/how much for the specialty grains for the cap? This will be my first partigyle, so any hints, suggestions, or corrections to my recipe are welcome.

Personally, you don't really need to worry about too much after you've collected you primary amounts of wort. The general college of thought, I'm sure you're perfectly aware, is to stop your runnings at 1.010 to prevent an excess amount of tannins from being extracted from the husks.
When I'm doing my calculations  in preparation to boil the session beer, is to dial back on the amount of hops uses.  But looking at you grain bill, you've got a sizable amount going on and I don't think you'll have a very weak second runnings. Keep in mind the balance between your alcohol, malt flavor, and the amount of bitterness you'll have. It would be interesting to know the OG of the second runnings to make the IBU choice.
Good luck with it.