It should be fine, knight gobies are fairly non aggressive and as long as the tank is big enough the F8s should leave them alone. What size is the tank?
The tank is a little small and there will probably be problems between the puffer and knight gobies due to territory requirements, i wouldnt advise this combination in less than a 36x12x15" tank.
Ok will wait until I get a new tank when my puffer outgrows it... would you recommend getting a couple more BBG's or should I just stick with what I've got? and I take it another f8 is out of the question!