For a boot drive you need to use SSD.
Boot from a flash drive would be slow.
You will need to install the excel program to the hard drive form the OS.
You can not say have the ssd with windows and open the excel program already installed from the other drive. I does not work that way.
You can have the excel worksheet files on any drive or flash drive and it will open fine.
As far as chrome, an ssd drive will help it open fast, but the browser itself being slow once it is already opened is RAM memory being limited.