You need three identical spikes/speaker, unless the speaker manufacturer suggests otherwise.
Most quality speakers (and the Helicon's are certainly that) have the issue of time alignment of drivers incorporated into their design, either by using a sloped baffle or having the tweeter slightly recessed relative to mid/bass divers. In this case, using different length spikes causing a back tilt may do more harm than good. Check the literature on the Dali's to see if the drivers are time aligned - I'd be surprised if not.
Thanks for the responses. I checked the manual and found out that the sound distribution and correct timing were taken into consideration during speaker design, so no need for the tilt is necessary. In this case I will stick with the same spike heights. As far as Dayton spikes are considered do you know what is the tread size (I need M10x1.5)? I would like to keep original mounting holes.