I have always seen ocellaris referred to as non-aggressive, and I thought firefish were a little territorial, but you are right to ask and do the research. I'm sure someone else can point you in the right direction.
Honestly, I don't see any problem adding the goby pair with respect to the timeline of the clowns, as they are supposed to stay at the bottom. It is the column swimmers you may want to wait on if you are trying to pair the clowns, for the reasons I listed above. But, i'm sure people try many methods.
Pairing the clowns can be a time of conflict for the tank, i'd just let them do that and then worry about the rest since your tank is small.
Best,
-A