Is it health for the motors to suddenly start then stop then start them? Or should I gradually decrease and increase the speed?
“Healthy” is a somewhat subjective term that will largely depend on the quality of the motor and the system of which your motor is a part. In general, the more violently you do something, the greater the risk of damage as a result of the higher forces/stresses involved, but I can’t say much more than that. The main benefit to gradually ramping speed up is that it can smooth out current spikes that occur on change of direction or a sudden increase in speed. This can be especially useful if you have your motor driving a load with a lot of inertia. Note that it’s okay to let friction slow a motor down.