If your design is for several fully parallel connected strings of 10 cells, (ie.
nP10S for some
n,) then The bms should work fine for some lower n down to n=1 for a simple 10 cells in series. (but be sure to to exceed the maximum charge current etc)
Whether the BMS can manage fewer than 10 cells in series depends on the BMS
Combining mismatched cells in parallel is less of an issue than combining mis-matched cells in series; because of lithiums cell's large voltage swing during across charge state combined cells will self-level (but be sure to get their charge states close before combining them so that the leveling currents do not exceed the maximum charge/discharge current allowed)
It's fine to start with a 20 or 30 cell 36V battery during development and then to add more parallel cells afterwards. The cells in the first battery pack will experience more wear in the initial stage which will reduce the useful life of the battery pack by some amount. but if it saves you form a hernia, or better fits your cashflow to work this way I think it's still a good choice.