Yes, you can connect two GPU's with different size memories together. And yes, the GPU's will be limited to the smaller of the two memories. Also, you will only get "about" a 15% increase in performance.
There are more bad reasons then good reasons for using Crossfire these days.
Mostly, game developers are no longer supporting it. (read some of the other treads about Crossfire in the forums)
In my opinion, a better option would be to sell the RX560 you have and get a RX570 or RX580.
AMD Radeon 500 series - Wikipedia
