refresh my memory...you used biospira right? Most places have fish guarantees, generally if a fish dies within a day or two of getting it you can return it to the store for a replacement. I would get two. Cories really need to be in groups...but yes they are spastic little guys and 'going nuts' is normal
For questions about the livebearers (mollies, guppies, platies) you should check out the livebearer forum. Lotus wrote up a great FAQ and there are lots of posts.
As far as plants go...also check out the plant forum...tons of info there so you dont have to wait for someone to answer your question in this thread
The otto is probably very hungry. You put him in a clean tank with no algae. You need to drop in an algae wafer, maybe a slice of zucchini or cucumber for him. The cories would love some shrimp pellets, and they'll eat some flake food that falls to the bottom.
I like to use Tetramin Pro tropical crisps as the staple for most of my fish.