Welcome to the forum, and hooray for soon-to-be new (to you) tank! It's good to ask these kinds of questions in advance, always nice to see people who take the time to learn!
You may not realize it, but there are a LOT of questions contained in your post! I'll try to address some of them, and I'm sure others will chime in with further information. . .
First, a 10g tank isn't considered to be very big, you don't have a lot of room for stocking. A good place to start would be to look the fish you're thinking about adding up in our Tropical Fish Profiles
, the link is located on the dark blue toolbar at the top of the screen, and sometimes a fish name will be shaded in blue on a thread, this will also link you directly to that profile.
Tetra and Guppies require different things from a tank. Though some people do keep them together, it isn't really the best for either type of fish. A good starting point when considering stocking is to try to figure out what kind of water you have coming out of your tap. Some fish prefer soft water, some like hard, and some need to be in a specific pH in order to thrive. These things can make a big difference in their expected lifespan, and overall health. Sometimes a call to the local water company can get you this information, or fish shops can run tests for you - if not, there are tests available that you can do yourself at home.
Along the same lines of water hardness and pH, Sand can
affect pH, but some sands are inert, and don't. Neither one is 'good' for the pH as such. . .because this depends entirely on the fish you choose to keep in the water! As a general rule, you're best to choose fish based on the type of water that comes from your tap - matching the fish to the pH and water hardness that you have available is so much simpler than trying to change it and keep it stable when you're starting out. Though many people do it, I feel it's much easier to learn with what you have. Once you gain a more complete understanding of how tanks work, then you can begin to look into altering things, if you wish to.
There is a LOT
to be learned about water hardness in the aquarium, and it can be a bit confusing at first. Take the time to read this article on Water Hardness and pH
, you will find some very valuable information in there! Feel free to ask loads of questions, it can be very confusing at first!
You'll want to put a fair amount of research into 'suckerfish' before you go any farther, too. The Common Pleco that you will see in many beginner's tanks are not suitable for small tanks at all, they grow to be very large, leave more waste than they allegedly 'clean,' and can be aggressive to tankmates. . .
Shrimp could do very well in a 10g tank, provided the water is right, and the tank established. However, they can be fussy, so again - more research into which specific type of shrimp you're interested in and their needs will be required before you decide.
I'd like to take a moment to direct you to our article on the Nitrogen Cycle
important information to understand when keeping a tank, and especially before adding new fish.
Hope this helps direct your questions a little bit! It can be a lot of information to take in at once, but taking the time now to learn as much as possible will go a long way to ensure that your tank is happy and healthy (without cloudy water which was likely caused by mismanagement by the previous owner, or dirty sand, impossible to know without more specific information) for you to enjoy for years to come!