We are planning something similar, but we are planning a wood/beam floor from standing dead timber. Walls with earthbags on rubble trench foundation with a bond beam, dirt floor. Vapour barrier under the floor/ceiling, floor beams, straw insulation, then vapour barrier on top of the straw under the top floor. I think if I was putting a chicken coop on top instead of a screened gazebo I would use sheet metal/roofing tin for the floor and have the floor with a slope(s) to the door for cleaning. But I don't see a problem with making your form for the cement floor right on your flooring/roof beams as long as you don't make your pour too deep. Think I might want to add a supporting post or two underneath depending on the span though. I would call my local building inspector and ask him though, he's a great help if I don't pester him too much! When my mom had tile put on her floor they just poured the cement right on top of the old subfloor, then the thinset, then the tile.