I have a couple of KJW P229 gas guns and shoot them in the house all the time. The biggest danger is breaking one of the old ladys knic knacks or lamps, or having her step on the BB's. You really do need to be careful of some things though. The bb's will dent drywall and other surfaces, and crack window panes, etc.
The gas guns are great for realistic practice, they work just like the real thing and while the recoil and blast arent there, you still have to track the sights as the slide cycles, so its pretty realistic.
I mostly use static targets for draw and shoot, etc, but fighting the kids and working on situations in the house can be very useful.
For my static targets, I use a piece of the blue house insulation with the "kill zone" of your basic IPSC target cut out, and a towel stapled to the top and sides along the edge. I leave the bottom unstapled so the bb's can drop out and I put a small pan underneath to catch them. The bb's hit the towel and drop out the bottom. I just staple a paper target over the cut out. If you should miss, the bb will just go into the foam. These are easily moved to set up pretty much any kind of situation you want, and allow a 360* indoor range.
They look like this....
The orange flagging hanging on the target is a visual cue that there is a live gun in my holster and not to shoot. You do need to pay attention, especially right after you have been shooting and switch out the guns. Most of my practice involves moving while shooting, and its very easy to just draw and shoot. The guns are so close, the only real difference between the them is the result of the hammer falling. For me, the ribbon is just another step that forces verification.