The pebbles will be easier for airation. The soil shouldnt go bad. Sand can get annoying, but plants can grow better in it as long as you airate the sand or get MTS, which I would recommend for you as well. They only eat decaying plant matter, they won't eat live plants.
I'm seriously considering digging up a patch of my yard to see what the soil looks like lol...I'm so tired, and I haven't even started this tank yet! I did manage to make my divider, it came out great! And I have a spot picked out, a table ready...I'm just so anxious to get it up and running! I used to live in California when I was really little.
Oh, yeah I would return it if you can. Miracle Grow Organic has bark, but you just sift it. Though at first you get this weird fungus type stuff from the wood. After a week or so it should go away as bacteria grow and eat it. Algae eating shrimp also love it.
I'm on the West, I live by San Jose, home of the Sharks.
If it is organic usually it does have some bark in it. Most people get some tyoe of softer or pasta drainer or even sometimes metal mesh and they sift it out. Just get the big pieces out. Or you could return it.