some thoughts:
the battery is the core of the system; if ti can't output the power to run the system, then the system won't perform the way you want it to.
either it will sag in voltage a lot from too high a load for it, which means the power will drop a lot, and as the battery discharges closer to empty it may even shut off long before the battery should be empty, beause the bms sees the voltage drop below the lvc low voltage cutoff and is trying to protect the cells. the cells will also heat up more than they should, and age them faster.
or the bms will just shut off to protect against the overcurrent load, turning off all power to the bike when it happens.
if the overlaod is bad enogh and hte bms can't shut off to protect itself, and the bms fets aren't made to handle that much current, it could damage or destroy the fets, requiring the bms to be repaired or replaced to be able use the pack again.
depends on how the bms is designed. the cheaper the battery, the cheaper the bms design usually is, too.
you can fuse the system at about whatever the limit of the battery's capability is, but every time the controller tries to draw more for long enough it'll blow the fuse. that's gong to get annoying real quick if it happens a lot.
fuses don't blow right at their current rating, either...their rating is generally the max they can sustain...it can take a lot more to overlaod them enough to blow.
they also need to be rated for the voltage they have to break, so if you have a 72v pack you need one that can handle the over-80v that will be fully charged. otherwise when the fuse tries to blow you may just get arcing inside the fuse instead of blowing the circuit, for as long as the current demand is high enough, and if it stays hot enough long enough the fuse or holder can burn or start a fire in stuff around it.
also, fuseholders for like the glass fuses and some of the blade fuses can be crappy. when they are, they don't hold the fuse very well, so there's a high resistance in the contacts, and they can get hot enough to melt the holder plastic, and eventually start arcing and even catch fire under some condtions...but usually the melting just causes problems with the system like intermittent cutouts, etc.
bolt-on fuses won't have that problem as long as you use hardware with locking nuts (nylock, etc).
if you can't adjust any of the controller settings, then if you really have to you can cut it's current limit down by disconnecting one or more of teh shunts in it, if it uses more than one. if it has four, taking one out cuts the limit by 25%. if it has two, then it's 50%, etc.
the controller is what limits current in the system. if ti's too big for the motor or battery, neither of those can do the limiting hte controller is meant to.
a smaller motor can take some mroe power than it is rated for, but how muhc depend son the situaiton and usage, as well as motor type and design. dd hubs ake more than geared, and usually for longer, before they have problems. hills and higher pseeds force the motor to draw more power, making it hotter, as do lots of stops and starts. if it's only short bursts of a few seconds at a time, with time to cool off afterward at a more normal power level, most motors can take more than twice their ratings, sometimes several times the rating.