We bought leather sofas as they were the easiest to maintain. Have you seen the Surefit brand of cover. May be pricey but looks like they are made to fit various shapes and sizes of sofas and chairs. Not sure if the slipping issue on leather will be resolved. Check the website.
To get any furniture cover to stay on leather is difficult. Try placing thin foam pieces on the arms and along the back before puting on the slipcover. You may have to use tape to hold the foam in place depending on whether the leather has a very smooth surface or a more textured one. I have mostly leather furniture and love it.I have three Bullmastiffs, a Chihuahua, a Rat Terrier, and two house cats and eleven grandchildren so my furniture takes a lot of abuse and still looks good.I have asthma and my husband has emphasema so we don't have carpeting or upholstered furniture.
I have leather and love it. I have cloth in the living room and it is harder to keep clean. We have 3 cats and one dog...(and 2 kids) I think the sand etc. might be harder to clean on the cloth. I use one of those sticky rollers and go over my leather couch every other day...works really well. Have you tried one of those? Another thing you might want to try is a waterproof mattress pad underneath a cover. They tend to be sticky on the back and might stick a little better. You can then apply your regular cover overtop? Just a suggestion. I know, my Max is lab and loves the couch...lol...
Yes, I have been using bath matts with rubber backing. They do not move easily, they wash up great and they come in all sizes and colors. I love the ones from Target by Fieldcrest. The rubber backing lasts longer and does fall apart in the dryer.