Change *.tpl using Miniportal

I´m running a community based on the miniportal script. I really want to change the way the portal acts when you are at the portal itself (frontpage) and when you hit the forum link. I´ll try to explain:

As it is right now the community uses main.tpl but I would like to use main.tpl and forum.tpl if it´s possible. So when you are at the portal page (frontpage) the community uses main.tpl and when you hit the forum button/link it uses forum.tpl with another layout. This means that I have the freedom to decide wich blocks/pictures and so on will be visible in the diffrent *.tpl´s. In the end if this is possible the layout could be changed like below:

Portal ---> main.tpl
Forum ---> forum.tpl
Search ---> search.tpl
Userlist ---> userlist.tpl
...and so on.

I hope you understand what I mean and please help me with this matter if it´s possible. You can find my site at www.cssforumet.se