If you're concerned about the fin rot spreading, I don't think it can. But if you still want to clean the tank...
- Use a 1-9 ratio of bleach. 1 cup of bleach to 9 cups of hot (hot from the tap) water. Pour into tank and scrub well.
- I placed mine in the dishwasher with NO SOAP. I run it twice just to make sure there is no soap residue from the first washing
- Place tank outside all day. I mean, all day. Even overnight if you can. That way if there is any bleach residue it'll evaporate out.
- Bring inside the next day and rinse thoroughly one more time.
As far as the gravel, soak it in a bleach solution as mentioned above, rinse well. Place outside with the tank on a cookie sheet in a thin layer to it all dries evenly. I would rinse one more time before putting it in the tank.
Placing the filter in the established tank is a good idea, that way you will have some BB to start out. You can also get a sock or stockings full of gravel from the cycled tank it put it in there as well.
I hope this answers all your questions. Good luck :)