There used to be a myth like rule of 1" per gallon for fish population. That's sort of silly. Fish that will grow large really need a large tank. And fish should never be crowded, so too many fish in a small tank is a bad thing. Also, we don't want to mix aggressive fish with timid fish that will be constantly bullied. I have a male swordtail right now that relentlessly chases any other male swordtails around/away. I guess he wants to be the sultan of the harem or tank boss. (He's not on my good side and may soon find himself at the LFS!)
Anyway, the real number of fish that any tank can support is really relative to how much constant effort (and sometimes $) YOU are willing to invest to keep a clean tank...as it largely comes down to water quality. If you feed high quality foods that result in less waste and you perform routine tank and water change maintenance, a tank can easily hold more inhabitants. If/when you slack off, even a larger tank with fewer inhabitants will soon suffer.
Now a 7g tank is on the nano side, but assuming you do the work, you might be able to add a couple more danios and/or another platty or two. But that would be max.
I'm not sure about the Siamese Algae Eater as I've never had one.