How does the process of nucleation work for boiling liquids?
Firstly, the average kinetic energy of the water has to be high enough (water obviously doesn't start to boil at room temp. and pressure the moment it is poured into a pan).

As the water gains more and more energy, small bubbles appear but don't fully rise to the surface as the energy of the gas particles is transferred to the surrounding water particles, eventually losing enough energy and becoming liquid again.

But if the energy of the gas particles is lost too slowly for this to occur they fully rise to the surface. This will only occur at a certain temp. difference i.e. the liquid has to be close to boiling temperature if not it will absorb too much energy and the bubble will become a liquid again.

A single bubble can increase in "size" (more molecules inside the bubble compared to when it started not talking about the diameter) as it rises, similar to the cluster effect you mentioned. This will only occur if the energy difference is high enough to vaporise the surrounding water molecules and not just increase their kinetic energy.

This can also not occur depending on how "violently" the water is being heated, or at which temperature the heating element is, therefore the temperature of the bubble being produced.