Yeah you can have 2x8 and 2x4 gb of ram in the same machine. Best to use the same speed and timings, even better to use same brand.
As far as going 24 or 32gb of ram, that is entirly on what your needs are and how bad 16gb of ram is crippling you. If you are not using more then 10gb of ram as is, then an upgrade will not help you one single bit.