In Marathon, the Fusion Pistol bullet and the projectile that the S'pht shoot seem to be pretty similar. Do they have similar damage amounts too? They both seem to be electricity weapons
This is kind of hard to answer, since the Fusion pistol's secondary fire is chargeable, so it does varying levels of damage. Plus, you can't fire the s'pht weapon yourself, seeing as it's kind of a part of their body. If you wanted to compare the damage, you'd have to figure out how much damage they do in single player, and then go into multi-player and shoot another player with a fusion pistol and see how the damage compares.
Also, the various difficulty levels in the game change the amount of damage you do and is done to you, so you'd have to test every iteration of the difficulty as well.
Therefore, the answer to the damage question is likely "it depends."
As far as the shots looking similar, Marathon originally came on floppy disks, so there wasn't a heck of a lot of room for game data - it's likely that they use the same model for both things.
Story-wise, the s'pht don't use the same technology precisely, but they are able to duplicate fusion batteries to an extent. Perhaps the technology is similar, but Bungie never expounded on this during the Marathon trilogy as far as I'm aware. The following is an excerpt from the Marathon 2 manual:
(This quote is referenced as part of a Wikia article on the Fusion Pistol)