For couplings I would recommend Greenwich couplings which are a fold up etch that have an arm at the bottom that is wrapped in a fine iron wire so that they can uncoupled by magnets in the track when the fitted vehicle goes over it. They are a little fiddly at first but it gets a little easier once you assembled a few pairs. As its the mine wagons you are taking about, for mine I have used identical couplings to the greenwhich made by a company called RT models which are smaller and were originally designed for slate wagons. Despite this, they can be used for the roco mine wagons if the single shank joining the couplings together is cut in half cut with both ends of the couplings fixed into where the original couplings are (seen in picture below). One thing to bear in mind no matter what coupling you use is that they ALL need to be at the same height otherwise they wont engage properly. A suitable coupling height can be found at RT models and, in my opinion, its a nice piece of cast resin that accurately does the job. As for fixed rakes, I would use fine blackened chain from Eileens emporium in the intimidate wagons and the use the rt models/ greenwich couplings at both ends of the rake. Hope this helps!
Dan