yes, try it!
I have short hair and I use clip in exstenions, I actually put them in upside down so when I pull my hair up it lays flat against my head , if not you get a weird bump in the back and if any baby hairs fall out I just bobby pin them , if you have any questions you can contact Me on my blog www.beautywithsarahc.wordpress.com
I think the little baby hairs will fall down at the bottom, but you could use a little bit of wax, hair spray or gel to get them to slick up into the ponytail. Bobby pins would also help you with this :) good luck xx
The only way to find out is to try it. But honestly, I think it'd depend on where you put your extensions to be sure as to if the baby hairs would fall down. Good luck. :)