Randall's Assessor in a 13.5

[Gatling02]

I've had a bit of trouble finding information on these fish but I love there coloration and swimming pattern. Would keeping them in a Fluval 13.5 be ok. I'd like to keep it with a goby, blood fire shrimp, and dottyback but I'd drop any of these fish to keep the assessor if its an issue of compatibility. 

[jambon]

I also am a fan of the assessors.I kept one in a 16 gallon with no issues. Due to the dark color it was mostly out of sight, hiding in the shadows. I also kept a yellow assesor which was more sociable and visible. I would drop the dottyback from your list as they can be aggresive. 

[Gatling02]

Thanks for the help, do you think something like a possum wrasse would work with it. I see that there peaceful and I wanted something active because at least in freshwater, usually having something thats always out and swimming around encourages those shy fish to start coming out more often then they usually would.

[A.m.P]

1 hour ago, Gatling02 said:

Thanks for the help, do you think something like a possum wrasse would work with it. I see that there peaceful and I wanted something active because at least in freshwater, usually having something thats always out and swimming around encourages those shy fish to start coming out more often then they usually would.

That's not a great strategy for marine fish in general, especially not in smaller tanks. Assessors are largely cryptic and will remain cryptic even with company, you can try to build an aquascape which encourages it to hover somewhere you can see it but it will still spend most of its' time belly-to-rock or upside down somewhere dark and sheltered.
 

IMO a 13 is pushing it for a possum wrasse and the bioload of those two fish would be a bit difficult to deal with, basslets are pigs and the wrasse will be grazing constantly. But it would probably be doable, perhaps something like a greenbanded goby or (if no sps are planned) a clown goby?

Here's a good guide for sizing tanks and fish, most new entries in each gallon class are intended for single-specimen displays and the list is a bit generous.