For leg warmers the suggested yarn would allow some elasticity so that the warmers maintain a shape after wear and washing. They won't get baggy and fall off in other words Cotton tends to get baggy btw so you can add that to your list.
Rayon will often get a shine to it yes.
I would tend to purchase elsewhere and use the suggested yarn if possible for this sort of item. It has more than (just) 50% rayon so that's a particular requirement and I would try and use another store however, if you can't do that a polyester blend around the same level may be a reasonable alternative. So, look for 51% poly and 49% wool and look to see in the explanation of the yarn whether it talks about stretchiness being a quality of that yarn. If it says that it will probably do a good job for those warmers.