Question 1:
It really depends on your preference. I personally like corydoras catfish, they do a good job of eating the food that falls to the bottom. They also keep the bottom of my tanks looking 'busy'. I would steer clear of the plecos with a 10g as they do create a lot of waste.
If you wanted more fish for the middle/upper levels of your tank, you could get away with having no bottom feeders. You should use a gravel vacuum to remove all the detritus and uneaten food.
Question 2:
Again, its all personal preference. I have a piece of decor I got from walmart a while back. It is two barrels, one on its side and the other upright. It had some funky colors on it a while back but now it has been 'aged' and is a more natural color. I have also gotten some java moss to attach to it. It is one of my favorite pieces of decor now. If it isn't too much money, try it and see if you like it. If not, take it out and toss it!

I've heard of people using coconut shells that are cut in half for caves. They cut a 'door' out of the half shell for the critters to get in. I heard good things about those and java moss attaches easily to the shells.