Give some thought to what caused those islands to form. An archipelago like that is probably similar in history to the Phillippines. Take a look at the distribution of those real-world islands (try Google Maps in satellite view mode). Notice that they tend to form chains--they're the peaks of an undersea mountain range. I think I'd expect to see a chain of large islands running southward from that big peninsula, possibly forming a rim over to that other peninsula to the west. The smaller islands would have formed between and around the larger ones.

To your question about which to do first: rivers or mountains. It's generally better to do the mountains first, because that will largely determine where your rivers should go. If you have a river that needs to be in a particular place, then you can put that down first and figure out where the mountains need to go to make it happen later.