No, sorry, you can't change the HP of a Pokemon.
When you get a Pokemon, they are given a random set of DVs. Hidden Power is determined from the DV of your Pokemon, so before you get your Pokemon, save first. So keep restarting if the HP isn't those of your choice. (I wouldn't recommend HP Fire on Sceptile, anyway.)
is a good place to learn about DVs, just search around the RS section.