hi, i don't believe there is anything to worry about, as long as the revs are around the numbers that dazmatic has suggested at tickover,

why do you need a bike to do less than 7-8 mph at tickover? its vary rare even offroading, yes in really tight places. try using the clutch to regulate the speed.

i know it sounds odd but a little faster and your balance is greatly improved something to do with the centrifugal effect of the wheels acting on the spindles, you don't need to know the theory just that it works[does for me]

i as gulgo, agree that for mixed speed riding keep the 15 tooth, if you mostly do really tight slow stuff a 14 or a change of bike is needed

my 2p's worth, stoic