setting the linear equilibrium's z value in the generic constraints helped me to counter gravity.
imagine a spring with a weight attached to it. you hang the spring so that the weight is at a specific height when the spring is in its neutral state, but the gravity streches the spring. so you've got to hang it a tad higher. Its basicly the same thing. Thats propably not the best way to solve this, but it worked for me.
if you are working with bone angles in your tails, you might need to tweak the angular equilibrium as well. Not sure which rotation axis is the one that controlls pointing up/down.
you could also set the linear and angular maxima/minima to stop the bones from falling down, but only if you NEVER want them to go below a certaint point in relation their respective parent bones.
edit: just saw in your xml file that you dont use linear movement at all. in that case the angular equilibrium values are what you need to tweak.
note that as you have a chain of bones you will need different values for each, as each bone carries the weight of all that follow. The earlier the bone's position in the chain, the more weight is dragging it down and thus the "higher you need to hang it".