I finally printed a flex object with a stock UM. My settings: repG35 & Slic3r. Using profile "Normal print", 25% speed and 230C. A lot hotter than expected, but it came out quite nicely... The walls aint really flexible, but the support inside is. Its a bit like molding rubber...
How is Flexible PLA different from normal PLA (besides being flexible?)? And where did you purchase flexible pla? Its like PVA: A horror for beginners. Its pretty flexible and bendable, and can be squashed together (so you need to get a really good force to get it working). And you can purchase flex pla from Ultimaker.