Some clarification, it's 500 connection per AP, meaning 250 per radio. MESH is only on the 5 GHz radio, and max number of association count if for non-encrypted station. With a single wlan using AES you can connect 180 station, and MESH uses WPA2-PSK, so in theory the max number of MEHS is 180, but no other client can connect on the 5 GHz radio at that point.
Monnat Systems is correct, keep the number of hop as low as possible, no more than 3 is recommended and the number of mesh link device as low as possible, consider the limitation that it is a shared medium, the more association the more overhead there is to consider. For example of you associate 180 11ac station you won't get (867 Mbps @ 80 MHz/2(overhead) = 430 Mbps/180 clients) 2.4 Mbps per station, given that all things are equal. If there are variation in client capability, distance, signal strength, etc. Theory and real world deployment do not necessary line up well.
As for MESH between protocol, 11n and 11ac should be able to mesh together. I have an R500 mesh to a 7363 which is mesh to a 7363 root AP.