You see, mIRC 6.2 already broke a feature in DCX concerning custom popup menus from mIRC (the status, channel, query and nicklist ones). The menubar one is unaffected. Khaled knows about it and said he would try to make it so that the old method is used again so we could regain the possibility to use this feature (we couldn't find a workaround so far).
On another note, mIRC can add whatever it wants to dialogs and it won't affect DCX because the DLL is completely independant of mIRC dialog controls. DCX manages everything on its own and has its own version of the mIRC regular controls because its easier to manage everything in a generic way inside DCX than to try to mangle around mIRC controls like MDX was doing. Overall, DCX commands don't affect mIRC handled controls and mIRC commands don't affect DCX controls (I think you could do some /did on DCX control IDs but the result would be unknown).
And you can use DCX controls with mIRC controls on the same dialog but there are limitations as using CLA for example which doesn't support mIRC controls.
Overall, anything that's added to mIRC is a good thing for users in the dialog space. DCX was only created to palliate the needs of people wanting more advanced features to fully exploit the available GUI in Windows. And the release cycle of bug fixes/new stuff is faster in DCX than mIRC as we all know.