2 dogs with digestive issues

Hello everyone and thanks for reading this. I am hoping someone can provide some guidance for me. I have 3 dogs – a14year old GSD, a recently rescued 6 year old GSD and an 8 year old greyhound. Currently I am feeding all three Hills for seniors because it has glucosamine for my older boy. I also mix in canned food with their dry and feed twice daily. My newly rescued girl has an extremely sensitive stomach and any variation from her diet or even treats will cause vomiting and diarrhea. Last week she vomited so I bought some cans (can’t remember the name but got at Petsmart and it was easy to digest). Well it was so easy to digest she had diarrhea for 2 days. My greyhound also has episodes of vomiting but it is down to about once every 3 months or so. My vet of course recommends Hills for GI but I am reluctant to try it because I’m just not crazy about Hills at this point. Now I have to feed all dogs the same food because if one leaves a little another finishes it up etc. It is just easier.
I live in a large metropolitan area so availability is not a problem, nor is price. I am just lost as to what to start looking for – could it possible be a grain problem? Any suggestions will be greatly appreciated
Thanks