RAM stick inter-compatability

I have an older motherboard with DD3 1066/1333/1600 RAM support with 8 GB (2 x 4 GB sticks, frequency unknown at the moment, but I will check) RAM installed on one chael. The motherboard (ASRock H97) only has dual-chael support (maximum 32 GB).

I currently want to upgrade this to 16/24 GB, but do not want to lose the ability to upgrade to 32 GB in the future. Is it okay to run 2 x 8 GB sticks on a separate chael (while the other chael only has 2 x 4 GB sticks)? Also, what if the 8 GB sticks are of a different frequency than the 4 GB sticks?