It's normal for some basic rules & regulations to be laid down by the developer - these are the estatutos (statutes), and form part of what is essentially the master deed for the entire property. It would also be normal for these to include details such as awning colours and outside lighting fittings, since this will ensure the building maintains a uniform appearance.
As I understand it, this document must be registered at the land registry in the same way as individual deeds, and any subsequent changes would have to be made through the correct legal channels (in front of a notary) and registered accordingly.
Section 5 of the Horizontal Law: The master deed may also contain rules and regulations concerning the exercise of rights and other provisions not prohibited by law regarding the use of the building, its units, its installations and facilities, expenses, administration and management, insurance, maintenance and repair, thus constituting a private statute that shall not bind third parties unless it has been registered in the Registro de la Propiedad [Land Registry].
For any amendment of the master deed the same requirements shall be observed as for its creation, except as otherwise provided regarding the validity of resolutions.
It would seem that since the first meeting has not yet been held, changes can still be made, but only if they are properly registered. Once the community has been legally formed, such changes can only be made by a unanimous vote at a general meeting:
Section 17: Unanimity shall (only) be required for the validity of those resolutions involving the approval or
amendment of the rules contained in the master deed or in the community statutes.
As for the existing light fittings not conforming to the new rules, that would seem to be one almighty faux pas!
To your second questions, all owners (not residents) must receive at least six days notice of a general meeting.
Section 16:3: The annual general meeting shall be called with, at least, six days’ notice and extraordinary
meetings shall be called with sufficient time to inform all parties concerned
Hope this helps.