I have interchanged the acorn and sombrero center caps on ralley wheels before and have never noticed anything different about they way they fit the wheels that would make one work and one not.
he's right. i had an acorn and sombrero style in my hand when i chose mine years ago. the acorn had too big of a diameter on the bottom that went into the wheel. it sucked since i wanted acorns over the sombreros.