Can action potential generating ion channels only be understood as assemblies of membrane proteins (and not as single proteins)?
Wikipedia's statement


  A voltage-gated ion channel is a cluster of proteins embedded in the membrane.


is most likely referring to the fact that most ion channels (maybe all? I hate to say "all" in biology) are made up of multiple protein subunits; usually these are all considered one protein's quaternary structure, though, so wikipedia's language is not ideal. 

I think you are then running with a misunderstanding of this statement to make some conclusions that are not warranted. Of course multiple ion channels come in to play when you are talking about membrane potentials at the cellular level. Of course you need to consider multiple ion channels when considering the overal membrane potential, but it is sufficient to know only a) the gating properties of a given channel and b) the membrane potential to know the contribution of that channel.

Action potentials are only roughly the same 'size' in terms of voltage amplitude - they are certainly not identical, and this size is because of the gating properties of voltage gated channels and the reversal potential for sodium. The membrane potential can never exceed the reversal potential for sodium via sodium-based conductances, so that puts an upper limit on action potential size. In addition, hyperpolarizing potassium-permeable voltage gated channels will open at depolarized potentials, further limiting the maximum amplitude of an action potential.

Ion channels don't exhibit any firing patterns: neurons exhibit firing patterns that depend on all the channels present and the physical morphology of the cell.