You can mix the fish up as much as you want. You can do 17 different fish, or 17 of the same kind. It's all up to you.

As for good long lasting fish... you're kinda out of luck. I would probably say that gwees would be your best bet, but they aren't anywhere near as good as dolphins were. I personally prefer monsters even though they don't last very long. They are good gold makers and people are normally willing to grab them.

As for how much gold you need, that all depends on what fish you want. I would suggest waiting until you have 17 of whatever fish you want to put in before you add any. Just putting in a few won't help you much at all.