There are a few points here.
1. You can have more than one texture per model, as many as you want, but you can't call them model#.jpg, they would have to have original names. One way round this is to make a texture map with all the different textures on it. Then you could call it model#.jpg, but it would mean remapping all the separate parts. I tried to do this with most of the models for Secret Worlds, but some don't work right, my well for one, because they need repeating textures.
2. I have put several objects together as one object before, I did it for my Stinky Wonder contest entry. It does mean you can have more models, but it still raises the kbs. I think I used the full quota of models and houses in that.
3. I already have lots of furniture models ready for use but never found the right way to use them. I have also made a room with 3 walls (the sides and back) and floor so that I could have a separate environment within the one level, but it doesn't look good, and as you move around you see round the edges. Plus that counts as one model.
It would be feasible though if the inside of the room was the entire level, then you wouldn't be able to go past the point where you can see around the edges, and the floor, walls and furniture could be made as one model.
4. Yes you can have things underwater. My waterlillies are lower than ground level to sit on the water. If they are used on dry land you can't see the leaves. I have several models of fish etc that go below the water, but again never found the right way to use them. They would be purely visual.
5. House are completely separate from models. You can make houses though. I have made one or two, but they have to be used as models. The thing with making your own houses is that they can be different sizes and shapes.
6. Reducing the polycount is feasible and I have done it sometimes, but as it reduces, the texture sometimes goes wonky, and you can only go so far before it looks bad. The answer to this is to remap the texture afterwards, but if it is a readymade model off the net, this is sometimes very difficult. Of course if you're making your own models from scratch then you can control the polycount so it wouldn't be a problem.
I have a couple of things in the pipeline half finished cos I ran out of ideas. They might get finished eventually.
