Is 60 series able to support ACS in AP mode? If yes, how to enable it?

Yes, 60 series is able to support ACS in AP mode.

To enable ACS mode, with summit supplicant, need to set "acs= 1" under network setting segment and it will scan first then choose the best channel to run as AP mode.

Here is an example of wpa_supplicant.conf with ACS enable,

# cat /etc/wpa_supplicant_ap.conf

ctrl_interface=/var/run/wpa_supplicant_1

network={

    ssid="miles-hotspot"

    mode=2

    frequency=2412

    acs=1

    psk="miles123"

    proto=RSN

    key_mgmt=WPA-PSK

}