Wemos Mini & Pro Micro Serial connection While using a 3.3v Pro Mini &amp; 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 Tx and Rx pins. Also since they are on the same PCB, GND pins are connected and RAW to 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 ).

What might be the problem ?
You don't need a logic level conversion between 5 V Micro Arduino and 3.3 V Wemos D1 mini. The 5 V ATmega 328p reads 3.3 V as HIGH and the esp8266 can tolerate 5 V logic level for tests. For long term connection you can add logic level conversion with a voltage divider to convert the 5 V of ATmega TX to 3.3 V of esp8266 RX.
RX/TX pins on Arduino Mini are Serial.
RX/TX pins on Arduino Micro are Serial1.
Serial is only USB on Micro.