firstly welcome to the board and i will try confuse u some more.
ok, koh tao, phangan and samui are in the gulf of thailand.
phi-phi and phuket are on the other side, in the andaman sea.
problem - 7 days. take 2 days out for return travel.
koh tao - be prepared to dive with lots of divers. unavoidable unless u choose a smaller diveshop.
samui - give it a miss unless u want maccers and starbucks.
pha-ngan - probably your best bet for party and diving. although the 2 do not mix, espescially if u go hard like everyone else in hat rin. if u do dive, make sure u go to sail rock and chumporn pinnacle. the other sites are not too great and nothing like u would have seen in fiji.
Phuket - good party (adult disneyworld) but maybe not the kind of party yure looking for??!! better diving if you go further out to the similians.
Phi-phi - good diving and decent partying. dodgy bars, thai boxing rings and reggae clubs.
unfortunately all the places u have listed are heavily touristed already. there are other islands u could dive on that are closer to BKK but the night(to-early-morning)life are not as good as the others.
here is some details about the 2 monsoons that affect thai diving:
The West Coast (Phuket, PhiPhi) should normally be dived from October until May with the first part better for clear waters (i.e. visibility) and the second half better for spotting large Pelagic. From June through September the monsoon blows in from the Indian subcontinent in the west.
The East Coast (Gulf Of Thailand - Koh samui, tao and phangan) can be dived all year, as the waters are not as deep and the sites much closer to shore. However November through December, the South East Monsoon can cause rough seas and make visibility unpredictable on occasion. The best months are from February through October as the water is clearest.
confused?? its alright cos i would love to be in your position...
good luck,
JP