My 2 cents would be to go ahead and try your 7 cell 1100 mah Nimh battery pack. It may work great for you.
The GWS flight pack for the Slow Stick has a 6 cell 400 mah Nicd pack with it.
If they are recommending that, then the 7 cell 1100 mah Nimh pack should work fine.
If it doesn't then you can always invest in another battery pack later.
If you want to have two packs right now. Then why don't you keep the 7 cell 1100 mah Nimh pack and purchase a 7 cell 600AE Nicd pack. That way you can fly with both and see which one you like better. The Nimh pack will give more flight time, where the Nicd pack will give more power. That way you can see the difference between the packs and decide what type of flying you prefer.
My personal preference is for power over runtime.
As a suggestion,
The KAN950 Nimh cells from:
www.eflightpacks.com
they look very promising for a Slow Stick.