Welcome to FishLore! It's great to have you with us! First off, how long has your tank been running? Is it cycled? If you don't know what cycling is, please go to the beginner's forum and click on the sticky I have in there. It has many articles that will help you immensely in the hobby.
Do you know what kind of plecostomus do you have? If it looks like mine, your baby can get over a foot long which is way too big for your tank.
I know you love your black shark, but he will also grow way too big for your tank, and being in a 10 gal. will probably make him very aggressive toward your other fish.
Your 4 mollies alone would probably just about overstock your ten gallon tank. My suggestion to you is to go ahead and make the investment of at least a 55 gallon tank. That way, you could probably keep all the fish you already have, and add those guppies you want. The zebra danio really needs buddies, but I'm afraid you're right that the one you have is a Jack the Nipper. Danios are aggressive, and usually when you get at least 6 of them together, they might leave the other fish alone in the tank, but guppies have those beautiful flowing tails, and I'm afraid the danios wouldn't be able to resist those beautiful tails. The pleco might also still be iffy in the larger tank. When they get that big, they have a tendency to creat havoc in a tank because they will knock things down, and they produce a lot of waste. They are regular poop machines. If you do have one like mine which is what is usually sold in most fish stores, I'd suggest you trade him in if you can for bristlenose pleco or 2 if you decide to get the 55 gallon tank. Even the bristlenose will be too big for your 10 gallon tank. If you get the bigger tank, your 10 gallon will be excellent used as a quarantine/hospital tank in the future.
It's great to have you with us, and as we get to know more about you and your tank, we hope you enjoy your stay here. We have a really good group of folks that want nothing more than to see that you are successful with your tank. Welcome to the family!
It's great to what them all swimming around with each other, and you'll be able to see their true personalities when they have some friends. 
