Most rock is safe for the aquarium. What type is really up to you. The slate has the advantage of being very stable when stacked. If you go with rounded river stone try to find ones that are more flattened and stack them in such a way that they don't slide off each other.
A lot of people here like to use eggcrate on the bottom of thier tank before adding thier rock. It helps somewhat to spread the wieght of the stone around.
Add your rock first making sure the structures are stable. Then add your sand. An 1 1/2" to 2". Add your water and conditioner, cycle the tank and then add your fish. :thumb:
There was a big thread about eggcrate usage before. Basically the outcome of the thread was that eggcrate did virtually nothing in the dispersion of the weight on the glass. All it essentially does is make the person using it feel better.
Note, this isnt my opinion, just the outcome of the thread. I'll see if I can find the thread.
this is something I disagree with. I think it came down to the miss understanding of the term "disperse the weight".
egg crate will not spread the weight of the rock across the whole of the tan (of where the eggcrate covers)
what it will do however, is disperse the weight in a very local area, so rather than have 1 point of rock pressing in the glass, you have a much bigger area presented by the 4 sides of the eggcrate cell instead. as such the weight is dispersed to some extent
there are other advantages as well. it will cushion any falls (to a degree anyway) stop the bottom being scratched (which would make the possibility of cracks in the bottom pane more likely)
the other aspect not touched on as often is that it "grips" the rocks, making them less likely to slide.
it is NOT needed, but can prove useful in the long run
with regard the OP question.
rocks first, make sure they are extremely stable, then add the sand, and finally the water, however make sure to pout it onto something (like a plate) to avoid stirring up the sand.
this should avoid any residual cloudiness after washing the sand (I washed 88lb for my 180, and despite stirring it up there was no cloudiness what so ever, however it requires patience when washing the sand)