I’m sure most people do this with soap pump packs anyway. I buy a couple with the soap in and there in after I refill with a liquid soap or shampoo. How did I end up using shampoo? There is a nice cheaper Australian owned and made shampoo that I bought to wash my hair, it turns out it was a good shampoo but made my hair a bit greasy before it’s next wash was due. I didn’t want to waste it. Next time my soap pump container ran out near the hand basin I put in that shampoo, it still washed hands, no one commented it was different and it didn’t get wasted. There didn’t seem to be any Aussie made hand soap where I shop, so I use this all the time now.
Another bonus about the pump packs is that if you fill it with the kids (kids being teenagers as well) soap gel it lasts a heck of a lot longer than if you give them a bottle or tube.
If you buy bulk shampoo decant it into a soap pump pack for the shower even if it is the more expensive fancy stuff as the pump is a good limiter, and gives you many more hair washes.
Don’t forget you can buy those fancy ceramic containers to decant your bulk hand soap.
My advice though is to use a cheaper shampoo that is hyper allergenic and a brand you know. I’ve been given and used some nice soaps, lotions, shampoos etc. over the years and broken out into some not so nice rashes.