When buying ram, do the sticks have to be the same?

[WhereHere1]

for example, on a board that has 4 ram slots, both dual. Would I need to get the exact same MHz and gb?Or could I go:

2x16gb (3600)
2x 8gb (3000)

Say from different manufacturers? Or would they need to be the same?

Also, would quad sticks work in a dual slot?

 

[WhereHere1]

So would you still get the full use of each 16Gb? It would just run at 3000mhz?