I think if your interest lies in agility, then do that. our club would train with a small dog that could do it, but there are going to be a lot of obstacles. One is the jumps with the dumbell, next is the bitework. Once you get off the rags and pillows and move to hard sleeves you'll have troubles. because of the short muzzle and smaller size already you'll be fighting an uphill battle.
I'm not trying to discourage you, but lots of people want to do schutzhund, but few have the stamina to stick with it even when things are "easy" If you just want to do parts that's fine, the OB Tracking and maybe tug work, but I think if you want to compete and have fun that way you'll be fighting an uphill battle the whole way and you should just be aware of that before you start.
if Agility is what you want to do and that is where your passion is, do it. you can overcome the dog reactivity stuff. If schutzhund is just something "else" you think looks fun, be aware that is a lot more involved than just showing up. If you want to do well that is.