Typically you're only steeping for flavor from those grains. Since you're using typically such a small amount it isn't feasible to count on 1 or 2 pounds of grain to offer that much fermentability.
I send most of my customers out the door with 2 pounds or so of caramalts and base malts in a bag together. I think the more fresh grain they can use with extract the better/more complex it will be and not so one-dimensional.
In summary, you're just steeping for flavor. Steep whatever you want to impart the flavor and don't worry about the fermentability so much if you're going with 90% extract. Hope this helps!
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