I doubt it will be fire hazard, people make battery wall with random laptop batt.
if you mix old and new they tends to drift voltage a lot when charging.
I’ve noticed 1 or 2 cells always have 0.1~0.3 volt lower and other cells gose 4.21 for 0.5 sec and come back sometimes, during charging.
It took me 30min ~1hr longer to charge and cells get touch warmer but there is no fire.
However, bms gets hot because longer balance charging time and that can be problem if the board is already hot after ride.
Also, battery cycle may be affected because of such irregular charging and discharging.
if you can keep the battery like 4.15 at full charge, I think it will be safe and the cycle improves. that is maybe way to go for you.
and interms of discharging with set of proper cut off, all the cells end at same voltage, with 0.02 voltage differences. again I don’t worry about it I’ve drained completely and charged right away after.
with 10s6p u will have to 120 ( with old pack may be110?) batt amp but i doubt that would happen often unless if it’s hard core mtb.
and 100mah is only 3% difference from original capacity which is already present in brand new batteries anyways I bet you wouldn’t even notice voltage differences occurs to me.