Here's how to get Android 16, Supreme Kai and Cooler/Metal Cooler:
Android 16:
Use Krillin's Dragon Universe (for AT LEAST the second time), and reach the Namek Stage. During this stage, at some point, 2 red dots will be on the map. Go to the red dot farthest to the East/right, and talk to a frog. After that scene, play the game as normal. You will notice, as a result of talking to the frog, the Android Saga will be a new part to the storyline. Now, search the islands on the bottom part of the map, and find the one labeled "Plains". Here you will find a busted-up Android 16. After you find him, go to West City and give him (Android 16) to Bulma for repairs. After that scene is over, Android 16 will become unlocked.
Supreme Kai:
Use Gohan's Dragon Universe (for AT LEAST the second time): view the two scenes with Videl and beat Goten. With these tasks accomplished, go to Kami's Lookout and defeat Piccolo. When the Dabura dot shows up, Go to the mountains on the Northeastern Coast of the Main Continent. After you find, and talk to, Vegeta: go beat Dabura. As a new part of the storyline, you will now go after Vegeta instead of Buu. Using the "Sense Auras/Ki Search" (R1), scan the surrounding area for a place marked "plains" (located near Central City). After the scene with Supreme Kai, he will become unlocked.
Cooler/Metal Cooler:
Use Goku's Dragon Universe (for AT LEAST the second time), and get to the part where you have just defeated Frieza. Three red dots will be on the map, and you must first go to the one directly below where you just fought Frieza. After you beat S.S. Vegeta, go to the red dot near the center of the screen. This will start the "Cooler Sequence". After you defeat him, a red dot will be in the same spot. After you talk to "Metal" Cooler, and BEFORE you fight him, carefully search towards the northern part of his space ship. There, you will find a capsule labeled "Cooler's Space Ship". This capsule is for "Metal Cooler", and it works the same way "Frieza's Space Ship" works,....kind of like a Senzu Bean, but with "Body-Armor".
Death is but a door, Time is just a window,...I'll be Back!!!!!