Telemetry DIY: Mission Planner Interfacing Guidelines/Docs

I am kind of doing a DIY Telemetry module, A kind of replacement of SiK Radio.
My 1st approach is to port the SiK Telemetry code to the new hardware.
While implementing, I had a doubt Is it mandatory to implement the AT commands?

My 2nd Question:
Since the telemetry uses 2400MHz band. Does Mission Planner has any limitation in using freq bands different from 433 & 900 Mhz.
Is there any good documentation on
Mission Planner and Q ground control interfacing with different telemetry modules.
Also Since using ESP8266, I can send telemetry via Wifi. Any guide lines on how Data can be sent over wifi