Fish and bugs will appear normally once you put back your rivers and such! You can completely change your island and fish and bugs will spawn normally.
Certain bugs will need flowers to spawn, such as butterflies. Some need bushes, some just spawn on grass. It depends!
The same goes for fish, some need the ocean (you cant really mess that one up haha), some a pond (a body of water that isn't moving in any direction) and some need a river (water moving in a direction, aka i think it has to have a waterfall or river mouth connected to it?)
From my experience it's there permanently unless you give them other clothes to replace it. The wardrobe system is pretty weird though so who knows haha
I'm not actually super sure but it would be weird if you didn't. If you're super worried about it I'd set your switch time to July 8th and time travel forward once you've finished the tutorial and maybe completed some tasks for tom nook (of course, unless you dont want to time travel)
I don't think starting on your birthday will impact anything though!
Good luck!! If you need any help feel free to ask me!