Is it possible to connect Cam and Video to the MicroMinimOSD with just the Video IN and Video OUT without soldering any grounds etc from VTX or camera ?

It works, as long as you connect any one ground (i.e. from the 5V supply). In some rare cases, you may get interference, but that usually points to a ground loop or dirty power supply somewhere else in the system.

Regardless of ground loops, I always connect the grounds on the camera and the VTX back to the ground pads on the OSD. It really does help reduce interference, as does making sure the video signals are routed as far away from ESCs, switching regulators and the PDB as possible.