I actually had this setup in my house sans the Draytek router but using Apple TC. Setup was TC and Extreme wired and both creating a wireless network (roaming). I have two Expresses that were setup to Extend.
Normal everyday use things were fine. However, sporadically when the kids were playing online games on the wireless network, the wireless network would go down. I couldn’t figure out why and only a hard reboot of the TC would fix it. A few phone calls to tech support and it was highly recommended that you don't combine the two types of topology unless you are certain that the extended network will not be in range of the roaming network.
I have since (yesterday) changed to just extended to see if that makes a difference.
Don't know if what I was told is correct or not but just passing the info on.