stocking questions??

[luvthereefer]

so recently i was given a 125 gallon tank complete with everything including fish.
They are a; one spot foxface
blue tang
sailfin tang
four stripe damselfish
cinnamon clownfish
niger triggerfish

the tank came with a bunch of base/live rock, but i think its just base rock with some coraline algae growing on it.
I plan on eventually going to live rock then inverts and corals.

but in the meantime i want to change my stock. I want to get rid of the tangs because they get too big, and the triggerfish isnt reef safe.

for example i was going to swap a tang for two young clownfish, would this be ok?? would the cinnamon be hostile? also would they nipped at by the larger fish?

is it safe to swap a large fish for two smaller ones, or would that upset the system??

or should i sell a fish...wait...then introduce new fish?

I really appreciate any awnsers you might have for me
thanxs in advance