You can make any icon you have uploaded your default icon. the only time you should not take an icon is if the icon maker has in bold letters, NOT SHARABLE, or DO NOT TAKE. Otherwise, if you credit, you can use the icon, and make it your default icon. All of my default icons I have used have been made by someone other than me. I don't make icons.
Always ask the icon maker if you want text added to an icon. I would never, ever add text to someone else's icon. Most icon makers are happy to comply.
As for QaF Bunnies. I would say to go ahead and post your thoughts. Even if someone had previously posted a bunny you are thinking about now, some writers may not have seen that post, so your post may spark something for a writer.