OK, the posibilities are near endless. Alright, they're not endless, but there still are a lot of choices you can make.
Getting a bottom feeder is a good idea. Plecos get too big for your tank, so avoid them. A lot of other catfish are nocturnal, like my spotted raphael. So, unless you like to sit in a dark room looking at a dark tank, you'll probably never see it after you buy it. There are, however, lots of good catfish that stay out during the day. I don't have one, but lots of peole are crazy about cories. Some of them are very colorful and they are more active than other catfish.
As for the shrimp, you could get some ghost shrimp. They make a neat addition to a tank, but you have to research how to take care of them. I had four and I liked them. But, I must have done something wrong because they all died within a week, so I don't think it was natural causes!
The frogs are cool. But, make sure you don't get a clawed frog, as they get big, mean, and will rip your other fish to little bitty fishy pieces. make sure it's a dwarf african frog. There is a thread about them in the general freshwater forum. Check it out!
You are at an exciting point in the fishkeeping experience. Make sure you stay calm and pick out a good fish. I know at least I get all kinds of excited when I am about to buy new fish. I learned the hard way not to rush out and buy a fish before I knew about it. There are lots of good fish out there. If you take your time, do plenty of research, you can find the fish that you (and your other fish) will live with and love.