You should be discussing this with your vet. Some meds do just fine being mixed, some you shouldn't give on the same day, and some shouldn't be mixed together. A fecal test should likely be done to determine what types of worms your dog has to see which type of wormer would be the most effective.
This is especially the case because you live out of the country. I doubt most people know what sorts of parasites are common in your area and as such what preventatives your dog needs. Around here heartworm prevention is an absolute must (basically a death sentence if your dog isn't on it), but that's not necessarily the case in all areas. I use the pro heart shot (currently on backorder for several months) and haven't had any issues with heartworms or other worms. My dogs occasionally get tapeworms from prey that they catch (eating an infected flea from a rabbit or something) but I treat with tapeworm drugs if those occasions arise and wouldn't consider tapeworm prevention as a necessity. Again, while opinions of folks online are worth considering this is something you should be discussing with a vet in your area.
