can i connect C5x in ptp mode to C5c AP?
If you have the PTP license for the C5x, probably.
Why not just use 2 of one or the other? (C5c or C5x)
C5x licenses have been disabled as of last night. No need to worry about license keys anymore.
With that said, yes you can connect a C5x to a C5c in PTP mode.
Does this mean that you will not need a PTP Feature Key
Correct. You no longer need feature keys.