First, measure the drop over the length. If it is 3" pipe, you need 1/4" or more per foot; if it is 4" pipe, you can get by with less, about 1/8" or sol.
If the pipe has sufficient slope, the lower volume won't hurt anything. In most circumstances, a single flush doesn't move the solids all the way to the street. As water moves by, say from the shower or the next flush, it moves it a little further.