Is my aquarium overstocked?

You're not overstocked for a 60 gallon. The Neon tetras would be fine.

Also some advice: get more of each tetra.

Edit: Yes, you would still have room for even more fish. Wilma is wrong, formulas rarely work.

Those are all pretty small fish and I'd hazard a guess that you have room for your brother's fish.

but for future reference here is a basic formula that has always worked for me:

for tropical freshwater tanks (cold water tanks and salt tanks will have a different formula):

(L x W) / 12= inches of fish (final adult size)

multiply you tank length (L) by your tank width (W) then divide by 12. your answer will be how many inches of fully grown fish you can put into your tank.

let's say you tank length is 60 inches and your tank width is 15 inches.

(60 x 15) / 12 = F (fish)

900/ 12 = 75 inches of fish

in this example if the adult sizes of all of the fish that are already in the tank totals less than 75 inches then you have room to put more fish into the tank.

again this is just a basic formula. some critters which would meet the size requirements of this formula might actually require more room to move around so always know what you're keeping and what they need to be happy.

edit:

not much room to explain here but the formula has to do w/ the surface area of the tank. so if you have an oddly shaped tank you'll need to measure the surface area of your tank to get an accurate answer for the formula.

Not really those are small fish. Just be sure their healthy. DB*