That depends on the linux distribution you use and i strongly recommend to read the documentation (i gave the link already). There are buch of linux installers (rpm, deb, solaris) and also tar.gz which can be just extracted and run.
You can specify the name of your server in the setup process. About linking it to your real domain name and all DNS stuff maybe Todd will help you.
What you mean by shared or reseller? I’m not american, so maybe it is not obvious to me. If it is a dedicated server, then it should have Java JRE installed or you should be able to install it yourself. Simple hosting won’t do.
You can create a group chat room and check the “Make Room Moderated” checkbox. This will mean, that every new user connecting to this room won’t has a voice right. Then admin or owner of this chat can grant a voice right. You can do it with Spark or with any other jabber client supporting such commands. In Spark you just right click a user and choose Grant Voice. This will work only for the current session. You can’t add a user to room members with Spark. You will have to go to Admin Console, Group chat, this room’s settings, User Permissions and then add a desired user to Room Members category. Other clients may have an option to add user to Room Members directly in the chat.
While being room’s admin or owner you can right click in the chat window while connected with Spark and select Configure Room. There you will be able to change some of the room’s settings, but i don’t understand what limits you are talking about.
There is no way to put a link to a chat on a website. You can use a SparkWeb, but it is not just a link to a chat, it is a web client, you have to login into it to be able to chat. SSL is not required, but if Openfire is using SSL, then SparkWeb will need it too. Of course, it is recommended to use SSL if you have such opportunity, for the security sake.