I do what Dan Pierce has done. I’ve changed everything to hook and loop couplers. They of course don’t look as prototypical, but they hold together for all day running, on my present uneven ground level track. They are also cheaper than Kadees, although the Kadees look great and are prototypical of course. I run my AMS cars only with each other with their couplers, but everything else, USA, Aristo, Bachmann, LGB connects well with the hook and loops. I am presently constructing an elevated railroad (if the snow ever melts), which will provide a more level and even track, (and I won’t have to crawl around on the ground any more!). When completed, I might consider converting to Kadees then because they look so great…if I have any money left after the benchwork construction.(http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-laughing.gif)
I’ve never paid much attention to pairing different brands together (other than AMS because of their size and scale). All the others look fine together to me. I use metal wheels as I have found that there added weight seems to help keep the cars bit more stable, and the track cleaner. But it comes down to what you want to do. It’s your railroad to have fun with!