Scenery on top of scenery

So, said to myself, self, now that you have added a 500g SSD and have a clean install of Windows 7 and a clean install of FSX-Steam how should i best proceed with addons. I have tons of scenery but I'm really not sure what is the properly layering process. I Have UTX and GEX for NOrth America and the tropics. I have FS Genesis North America mesh. I just downloaded but didn't install this new stuff that was released in the last week or two. I have a great many pieces of Blue Sky Scenery and I have ORBX PNW. I understand that when running, FSX picks up the last (or lowest numbered) scenery first is that true? I also have tons of individual airports and of course many of them them are dups I'm sure because Blue Sky for example does all the airports in their areas.

Could someone take a few minutes to splain to moi, for example "if you have x you don't need y but telling me what X and Y might be the recommendations. I appreciate it. I have been simming for a long time but I'm not a techie and i don't understand all the subtleties and I'm 72 tomorrow and can't always recall where i put my keys last. Thanks.