There is really no such thing as a “spindle bit”
There are router bits.They are made for wood. You can also use them on aluminum or brass if you take it real easy with very light cuts, aren’t too concerned about surface appearance, and you’re not afraid to throw a bit away after using it just once.
There are end mills. Endmills are mde for metal. There are generic end mills that will cut anything in a plain old sorta way. They will cut wood. There are endmills with flutes that are configured to cut really good on aluminum, or non-ferrous metals, or stainless or unobtainium or ???.
There are v-bits for engraving that will scratch a deep line in a surface, but won’t cut a groove through warm pudding.
There are drill bits.
If it fits in the spindle, and it is made to cut the material you want to cut then you can use it.