My two cents worth. (And hey, if it's good advice, you can pay me more than that.) :)
1) I've seen pictures of people's hives on long tables, and the hives were right next to eachother. I don't know how well it works though. I'd personally give them (and you) about 6 inches. Think about this - how easily can you work a hive, or take off/add on supers if they're so close they're touching?
2) I'd use wire if I could afford it. But wood can give it strength and stability too.
2) I wouldn't use wire any larger than the 1/8 inch - personal opinion.
3,4,5) I would (personally) use slanted, individual landing boards. I think the slanting actually helps them "glide" in better. Think of flying in - would you more easily see a board that's tilted to your face? or just a flat runway?
I'm nearly done with my two hive - screened bottom board hive stand. It's 12 inches off the ground. Has boards on the ends to hang a few frames (2 or 3). Slanted landing boards. Red painted lines and arrows on the landing boards (maybe overboard - but I'm a girl, so we think of silly things). Screen is 1/8 inch, no board under it, but I have slats to slide a board under there in the winter to cut down the flow (I could even use foam board to insulate if I wanted.) The hives will be 6 inches apart, but the space will be basicly a hole rather than solid flooring. I'll put pictures up in a couple days as long as the rain doesn't start up again because one of the pictures just HAS to be of the hives actually ON the stand.