My first thought is that it would be a very good idea to consult an experienced strata lawyer about this.
If you do go it alone, however, I would make sure there was a clause that says that anyone who installs air-con without permission or does so with permission but in breach of the conditions set out in the by-law, will have to remove the unit and reinstate common property at their own expense.
I would also create another by-law to cover downlights but, even better, have a look at this document which defines who’s usually responsible for what in a strata scheme. It is not legally binding but you can use it as a template for a catch-all by-law that covers all sorts of things (and the by-law will be binding). Change it to suit your scheme but in general it will stop a lot of arguments about who is responsible for what in your block.