Stacking window management mode is available at the window group level, on a group by group basis. Select the group you wish to make stacking, and press M+s or run the command stack on. Any frames in the group will disapear. Other groups will not be affected.
Floating windows can be moved using M+Mouse1 -- that is: hold down the modifier key and click the left mouse button on the window -- and dragging the mouse. Floating windows can be resized using M+Mouse3 in the same fashion. Click-to-focus still works.
Most of the frame manipulation related key combinations and commands are disabled in stacking mode.
No, there is no way to float specific windows while in tiling mode.
To switch the group back to tiling mode, press M+s again or run the command stack off. The group frames will be recreated as they were before the mode change.