As gbn indicates, it's not easy to change the built in report server method of presenting the parameters. SSRS likes to always use two parameters per line, presented in the order that they exist in the report (which must match the dependency order.)
So the alternatives that gbn mentions: Both involve building a "Wrapper" application: some custom code or a web page that you can code however you like to get the parameters. Then you call Reporting Services, either in code or by passing a formatted URL with your parameters. The report can be displayed in a frame, new window, or passed as a stream to where ever you'd like.
The URL access is pretty straightforward and reliable: I often use it either by hand (to create "favorites") or in code.
http://msdn.microsoft.com/en-us/library/ms153586.aspx
For what you are looking for, these might be more work than you expected, but they will be extremely flexible for your interface.
Jamie
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