Im not a expert but i can tell you from experience... sand will eventually seep though the Eco complete and mix with it.
Smaller particles will always drop to the lowest portion...
And unless u dont like the looks of eco complete being black, which then u can go another type of substrate, just have a single substrate.
If you want a beach effect.... half your tank 1 substrate and the other half another, you need a divider so they wont intermix.
But yeah i didnt do research the first time with eco complete and mix'd it with sand to create a bicolor effect... well needless to say that didnt last very long.. :P
Also no need to wash it... just dump it inside your tank... add water, run a filter.. and start your cycle...
Lastly, i suggest u get some root tabs... and plant them inside your substrate after u have cycled.
Eco complete does not all the required ferts inside it like AquaSoil does.
Its a very high CEC substrate, so root tabs work really well with eco complete.