1.)Will you please help me?
Yeah.
2.)If i do a stage import, is it just like a character, where i need to replace just the model + textures?
Yes, though some models need extra things to function properly (some texture animations, etc.) Just import the whole BRRES file into the stage.
3.)What are stage collisions? I assume that they tell brewl what is a solid object?
Yep. They contain data on walls, floors, ceilings, and ledges. They can be edited in the brawlbox collision editor, of course.