First, I would not add anything more to your tank. You are already well over the max bioload for your tank. An 8g tank should have no more than two very small fish. Keep the clown goby, take the other two back to your LFS and exchange for something like a tailspot blenny which is also a small fish but with a great personality. And do not get a catalina goby as they are not a tropical fish. They need colder water.
You also need to do more investigation of the fish you buy. Two of your fish are not suitable for an 8g tank. The red scooter blenny is not a blenny at all but really a dragonet which is the same family as the mandarins, and like mandarins, red scooter dragonets are considered a very difficult fish to keep. They have very specific dietary requirements. They must eat constantly throughout the day meaning that your tank must have a sustainable level of copepods which is nearly impossible in a small tank. You wil need to be constantly replenishing your pod population which will get expensive fast. If you do not do this, the chances that your dragonet will starve to death are very very high. If I were you, I would look to take the fish back to the store. Also, a yellow assessor basslet is not suited to a pico aquarium either. The tank is simply too small for the fish. They give minimum tank sizes for fish for a reason. Check out liveaquaria.com. They are extremely accurate in their tank size recommendations for fish. They also list how hard/easy a certain fish is to care for. The red scooter dragonet is listed as difficult so is not really suitable for someone newer to the hobby...especially in such a small tank.
