Are Neon Tetras, Harlequin Rasboras, Panda Cories, and Marble Hatchetfish compatible?

yep. all these fish will be good together. your amounts are fine and you'll still have plenty of room for more fish. in a 75 gal you may want to add more cories eventually but thats the only thing, as they help clean and so plecos.

just dont add all these fish at once. i'd add five or six every week over a period of time so you dont freak out the fish or shock the tank.

Yes - no problem with amount or compatibility.

Remember to add fish slowly!! In a tank this size - no more than 4 or 5 at a time until you reach your capacity.

Capacity is 75" of fish provided you have adequate filtration for this size tank and do regular weekly water changes.

Marble Hatchetfish

yup just make sure you have a cover because the hatchetfish are known for jumping out of the tank

They'll be fine together, however you will need a hood otherwise you will have Hatchetfish jumping out.