1st question possibly depends on the engine the model is destined for.. some engines dont like open meshes. But here ive seen models combined. 2. no idea myself, but i wouldnt quite rely on a pretty render to push your work.
If you are going for realtime 3D game models I personally would not present them with renders such as Mentral Ray since these are far from how your work would look in a game engine. Go for some hardware render such as Marmoset or similar imo.
What Flynn said, depends on the engine you are bringing it into, Unreal 3 I know watertight will always be better. - About the positioning of the 2 models, 1- What are the models you want close together anyway. Merged or not depends on a few things: 1- How close will you get to the objects / what angles the models viewable…