Trod
The indecisive one
I have a yellow assessor, been in my tank for a few months, could I add a Blue assessor, done some goggling and its a bit unclear as to whether they would get along, they can be kept in small groups but is a couple to smaller group
I also have
purple firefish
2 clowns
mandarin
Tank is 120l I know I'm pushing my stocking but I want one
I also have
purple firefish
2 clowns
mandarin
Tank is 120l I know I'm pushing my stocking but I want one

ive ordered one for my new tank £70!! i was like
but in your heart you know he is going to say the yellow is too teritorial and has been in there too long
now if you had put the blue in first 
Sis is right on this one, it's gonna be tough with it in there for 4 months. You could try to re-do the rockscape, thus destroying the current assesor's territory which might lower aggression when adding th new one, but if I can be honest, I wouldn't try it myself.