Ok, hopefully some more experienced members will chime in. I'm still new at this too, but there are a couple of things I do know from researching, so I'll try to help out. The rule is that you want to have no more than 1 inch of fish per gallon. So, for a 10 gallon tank, this means you can have 10 inches of fish. So, this is where you stand now:
1 Leopard Pleco: grows to 10 inches (So, you'll definitely want to re-home this fish like you mentioned)
Giant Danios: grows to 6 inches a piece (you'll definitely want to re-home these as well)
Two Tetras: currently 3/4 of an inch, but depending on the species may grow up to 2.5 inches a piece (these should be ok to keep)
Do you know what kind of tetras you have? This will definitely help in determining what future tank mates would be suitable. I'm pretty sure that most tetras are peaceful community fish, but Serpae Tetras (for example) are semi-aggressive.
At this point in time, I wouldn't recommend adding any fish until you've re-homed the Pleco and Danios. Although these fish aren't large now, they'll grow quickly and you don't want to end up in a situation where you're overstocked. Crowded fish can develop aggressive tendencies even if they don't have aggressive temperaments. Also, once you've re-homed the Pleco and Giant Danios, this will really open up your choices for you.