My purchase of a C model RC fell through. Can a GL858A from a Matrice 600 be used with an I1?
Yes, any of the controllers will work with any of the aircraft.
Honestly, here is how *I* look at it, based on everything I have read and know about the tech:
"A" controllers are originally sold with T600 aircraft. Everything is paired and it works well. Reference standard.
"B" controllers come out as a redesign with slightly lower power.
"C" controllers come out as a higher powered design for use with the T601 aircraft (v2/Pro/RAW) that has also a higher powered radio in it. Power is so much that pairing "C" controllers for Master/Slave has "issues" with the 5.8GHz feed used between controllers and thus issues ensue (primarily because they 5.8GHz antenna is the left antenna with part of the 2.4GHz system (can't remember if it is receiver or transmitting antenna), thus you end up having issues if two "C" controllers are used. The "workaround" for this is to pair a "B" controller "slave" with a "C" controller "master" to negate the issues.
Now, the higher power aspect was totally "cut off" by firmware updates from DJI, so much so that basically all three versions have near-equivalent power output with the latest firmware.
Ultimately, you can use a "A" controller with a T600 or T601 aircraft radio, same with a "B" controller and a "C" controller. They are all the same frequencies and protocols.
But, personally, the "C" controller as master with the right firmware on a T601 radio aircraft with the right firmware and you have a VERY powerful, long-range setup.
I think the majority of issues came into play with "B" controllers being used on T600 and T601 aircraft with "latest firmwares". Even then I know plenty of people using "B" controllers with the right firmware and having zero issues.
Ultimately, for my use I will probably keep the two "A" controllers with the V1 I have as master/slave. I will use a "C" controller as "master" with my V2 and use the "B" controller as a "slave" to the "C" controller.