While using a 3.3v Pro Mini & Wemos mini to communicate via Serial was successful, when tried with a 5v Pro Micro it fails.
To overcome different logic level I used a 10K + 20K resistors to lower Pro-Micro's 5v to 3.3v successfully to
Rx pins. Also since they are on the same PCB,
GND pins are connected and
5V at wemos mini is also connected. At the moment, it is powered up using MCU's USB input ( tried to connect to Pro-Micro only, Wemos Only, and both ).