I have tried short, medium, and long cage derailleurs on Birdy bikes. The medium cage derailleur doesn't have any issues if your chain is the right length, and you are running taller tires. When I ran the long cage derailleur the cage reached to the brake surface of the rim, and could catch things on the ground if I wasn't careful. I like to run a mountain bike rear cog on my Birdy as it makes climbing the steepest hills quite easy, and pulling a trailer with my daughter in it. I am running and XTR cog with a 34 tooth ring.

I now run a medium cage XTR derailleur with a set of 20" wheels, and have no worries, there is plenty of ground clearance. I run Panasonic slicks, which are quite narrow, but with 20" wheels the cage isn't going to come close to the tires anyway.

For a tensioner, I found a spring-loaded parallelogram type which bolts around the bottom bracket, with a jockey pulley just below and behind the chain wheel, it works perfectly. I couldn't easily fit the original tensioner to the new XTR derailleur, so I needed another option.