I think that'd be tough there. There are a bunch of lodges (hotels) and restaurants, but the fishing on Grand Roque seemed pretty limited to me. There's great fishing at the end of the runway, and there are really big and smart bones by the docks to tease you, but you'd want to get around too. There are plenty of captains to take you to other islands, so you can get around.
As far as the exchange rate, that really varies. It's anywhere from 4 Bolivars to 1 dollar at the airport, up to 6-7 to 1 from the guy that drove us from the airport. He said the airport had the worst exchange.
Flies. Gummy's floating and regular. Lots of them and use 20lb tippet. And gotchas and small deceivers.
As far as night life, it seems like there was a lot on Saturday night, but it was the last day of their busy season. The rest of the week was quiet, but there was still a lot of partying going on at the clubs. It's a nice island and the food at Acuarela was freakin awesome.
I've only been there once, there are others here who have gone a bunch. Their advice will be better.