ok, with regards to having a big fish in a smaller tank, this is a bad thing for the fish. Small tanks stunt the growth of big fish, even if they haven't got so big yet. What happens is, the outside of the fish will be stunted but the insides of the fish will keep growing leading to a very painful death. Yes it will be a while before your plec gets that big, but ideally it needs to be in a tank that is big enough for it's adult size. the other issue is, although the lfs said they'd take it back, there's no guarantee they will, they often just want you to spend money in their shop!
I'm sorry to say, but the nicest thing you can do for your plec is to take it back to the lfs now, and get a species of plec small enough for your tank.
Here is a link to
30 plecos under 8 inches
There are some lovely species perfect for you, and IMHO most of them are better looking than the bigger guys!
Your missing tiger barb might well have been eaten by the others, so no worries about not finding it, just keep an eye on the ammonia for the next few days and water change as necessary.
What is your pH? Mine is 8.0 so really high too! There are ways of lowering it, try searching for other threads as I know there is some tricks you can do with peat in your filter, or you can use RO (reverse osmosis) water to help soften (and lower the pH of) your tank water.
I think the worst thing you can do if you want to lower your pH is add any chemicals, as they tend only to be a temporary measure, will cost a lot of money, and most importantly not keeping the pH stable will seriously stress your fish. Often it is better just to keep you pH as it is. fish do much better in stable water conditions than if they constantly change.
Most fish you buy at your lfs are bred locally and are used to the pH of your local water. Was there any reason in particular why you wanted to lower it?