I have had a lot of success with gluing PLA and ABS parts together with Simple glues like Super Glue and the glue you get in model kits like Airfix but have not tried with flexible filaments. The flex filaments I have are PLA based and cannot see why they would not glue together.
Another technique i use for fixing parts together is welding with a 3D pen. You can get a 3D pen for as little as £15 from eBay and Amazon. This would enable you to use the same filament and physically weld the seems together just like welding metal. The join is incredibly strong and would work with every kind of filament that you printed the parts from. I have then successfully sanded down the welds (PLA / ABS) with a dremal, hand file and sandpaper.
The welding with a 3D pen technique is also very handy for adding small details or even fixing breaks / cracks / holes. Or even just to thicken an area for added strength.
I hope this helps