May i know is it safe and ok to always remain the smart pen attached to the mi pad? Will it overcharge?

I might not be the right person to advise you on this. Cos I don't use the smart pen. I think its too costly and I'm not a designer by profession or like to write or draws.

Usually such a device, there should be built-in overcharging protection to minimized the return rates. It can be assumed that the smart pen has such overcharging protection and the tablet being programmed not to supply charge to the pen once it has been fully charged to minimized the power draw from the tablet.

Hope this might answers your concerns. Just my two cents.