Are you going to make all of the Colossi? That would be an awesome mod!
Unfortunately, the maximum size allowed by DF's programming seems to be 200000000. Going over this limit may cause unwanted behavior.
You're going to have to scale the bigger ones down, I'm afraid.(There may be a way of getting around this limit using an interaction with a body size modifier syndrome. I'll check if it works.)
The good news: If you make the balloons a custom body part with the FLIER tag, it will fall when they are damaged. You can also make weak points with MAJOR_ARTERIES.
If you want to suggest that killing Colossi has a negative impact on your soul, there may be a way of doing this by giving their blood a syndrome effect. Might want to hold off on that unless you plan on making more Colossi. It would probably be very complicated to make it come out right.
You might also want to give Phalanx FLEEQUICK to increase its benign-ness. I think megabeasts might be pre-programmed to attack towns in worldgen though. I guess you could say that it's just passing through and happens to destroy stuff by accident
EDIT: Turns out that using interactions doesn't help get around the size issue. Also, the upper limit isn't 200000000. I'm not sure what it is, but it's much smaller... I have a creature that works at 33500000, but going much bigger than that starts to introduce weird physics, mostly it seems to loop around and go back to zero, but sometimes you get weird bugs like creatures that can't deal damage and all attacks against them pass right through.
Anyhow, Colossus-sized creatures will break DF's system, sorry. Gonna have to scale down, I'm afraid.