I found out the hard way LOL...

YOU MUST OWN THE STUMP THAT YOUR ROO IS ON BEFORE YOU COSTUME IT, A FELLOWSHIP STUMP OWNED BY SOMEONE ELSE WON'T WORK.

I had not realized that a fellowship stump doesn't let you rebuild a roo, which makes sense given the way SL perms work. If it did rebuild the roo would become the stump owners and not yours anymore so, of course, they don't allow that.

Thanks to Masquerade Ball for not making me feel like a dolt who didn't read the notecard hehehe. I figured I would post it here as a heads up for others who don't tend to read the notecards! :-)