It would probably be a bit overstocked but it depends on how many water changes you do and how much food you put in the tank. It also depends on the species of blenny. Some stay small (under 3 inches) and others can reach 6 or more inches in length. The dottybacks are pretty small and don't put a huge biological load on the tank.
Water change and gravel clean should help remove the red (blue green) algae. After the tank has aged a bit the red should go but you need to try and get rid of it early on so it doesn't stay.
