I was wondering if a person can silicone rocks on the back wall of a tank. I'm thinking of using a flat tyle of a rock, like slate or something.

I want a three-D back-ground with out the cost of the synthetic ones I have seen online.

Also, if I were to find my own unique piece of drift wood, could I disinfect that and use it, in the same exact manner if I were to buy one?

I would worry about the weight of rocks being siliconed as well as the possible ramifications if one were to come loose and smash into the bottom of the tank. I've known of people to make those backgrounds out of styrofoam though.
You can use any wood except for certain things like pinetrees and other needley ones I think. You just have to soak it for a long time usually to get it to stay put. Be careful where you get it from, some people may be spraying pesticides and such in your neighborhood.

I would be affraid to place the rocks in any way they are not supported because if they do fall they can break the glass or kill fish. As for the drift wood , I have several pieces I have gotton from a creek and they are great. I do wash them and get as much of the soft spots off I can. I have several pieces and if you collect the ones that have sank already you do not have to hold them down with anything as long as you dont let them dry out. They get watter logged and will stay on bottom but when they dry out take them and get a peice of slate rock and drill a hole and place a wood screw in the rock to the wood and this will hold it down. If you water turns a little yellow there might be some soft spots you missed and net to scrub.

I would only silicone rocks that would be resting on the bottom! Silicone tends to weaken with time and the weight of the rocks would make this happen even faster and you would not want one smashing to the bottom and cracking your tank! That would not be so good!