Most company's 2000w Fresnels can be lamped down to 1,500w and 1,000w.
Just buy whatever model fits the Lightbank "O" rings that you already own.
Unfortunately you can't "lamp up" your 1000w fixtures.
Your instincts are correct also. I always prefer using larger lens fixtures, as the quality of the light is so much better.
Even though "Baby" 2k's and "baby-baby" solarspots are very convenient, the quality of the light is much harsher.
In many studio applications I've actually used 5,000w Fresnels as my instrument of choice -- and lamped them down to 2,000w when necessary. Depending on how big your Chimera Lightbanks are you may need the output of a 5,000w source -- especially if you're using 30 or 60 degree "hard" honeycombs (which is the only way to go..... in my opinion!).
The bigger Lightbanks also afford you the possibility of using them on your HMI's.