Hi Jon,
I would suggest you install this to another directory e.g. forum which would then make the forum address
www.mydomain.com/forum. This allows you to put other things on your main page to give people the choice of where to go in your web site hosting but if you ever did want to have the forum as the main
website hosting, you could then put a redirect in place.
Regarding the ftp, you can add different users which have access to different folders of your site. With your account you can have an unlimited number. You will need to know however there is no anonymous ftp which means all users will have to login.
Hope that's helpful. If you want anymore info, just post back on here.