Health Canada makes decisions on what vaccines are approved and another organization called NACI makes guidelines (which are not rules but only recommendations).
The actual vaccine rollout is being left to the Provinces -and our local politicians & public health officials are all over the map regarding eligibility:
-Here in Ontario, Astrazeneca was approved for people over 55.
-Then approved for over 40 (my age group).
-Then the (extremely rare) blood clot issue surfaced and the Ont govt did a knee jerk reaction and halted all Astrazeneca leaving those of us who already had a first dose hanging.
-They then announced that the second dose might be substituted with an mRNA vaccine.
-Then there was backlash from portions of the public as the studies on vaccine mixing are not complete, they only show that mixing is safe and MAY result in a strong immune response - but no clinical data yet.
-So the Ont gov't is now saying Astrazeneca patients have a choice of our second dose-Astrazeneca or mRNA.
-And now NACI (the National advisory committee on Vaccination) is recommending mixed doses (Astrazeneca followed by mRNA) based on the preliminary vax mix study results mentioned above.
It would not surprise me that mixing vaccines does turn out to be effective (and some people are choosing to do so as is their prerogative) - but IMHO based on what I read, the vax mixing studies NACI is quoting at this time are smaller sample sizes and not 100% complete yet though they appear promising.
Because I would rather stick with what has been tried and tested in the UK & elsewhere - and before the Ontario gov't changes the rules once again - I chose, booked & received an Astrazeneca second dose as soon as I became eligible (after 8 weeks from my 1st dose).
Sorry for the long post, but that's as condensed as I could make it.
As you can see our wonderful conservative govt here in Ontario has been nothing but forthright and timely in their communications with the public, allowing us to make informed stress-free decisions about our health, and not on-the-fly at all 
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