Is it possible to guess a reasonable Kp and Ki based on motor parameters for speed PI control
It really depends on the load.  Your load can be divided into two parts: the inertial load, requiring torque to change speed, and the load that is doing work.  Decide where you are going to be speed and torque-wise.  If you experience a sudden load increase (the maximum you expect), you can calculate the corresponding drop in speed. How quickly can you supply the torque to accelerate and catch back up, with the maximum available torque?  If Kp is too large, you will overshoot on the voltage and experience high currents.  So size Kp to give you an acceleration that won't blow your torque budget. Then you can play with it.

If you are running without braking, you may require two time constants, because if your load suddenly drops you may coast from your inertia, and thus require different constants for increasing and decreasing speed. The rate of decrease will in this case not be driven by the voltage and so Kp must be smaller for decreasing speed.