Formations are as strong as their weakest link, it's not something to be filled with throwaways. The pillar character formation skill determines the maximum formation size and load, with small formations it doesn't need to be that high.
In the case of formation size, having more skill in formations gives you increased stability (a good thing since it collapses when it reaches zero).
In the case of formation load, having more skill in formations reduces the formation cost (Qi per second to have the formation activated).
Since there have been only shallow guides on formations, I'm in the progress of writing an expanded one. Auxiliary cultivators support the pillar cultivator by activating the auxiliary nodes (the hexagonal elements). The nodes themselves strengthen the formation in 3 ways.
First, there's auxiliary links, the characters (symbols) on the side of the node, when connected to each other give a bonus to the pillar (eg. 剑 – Jian gives bonuses to artifact mastery)
Second, enough auxiliary links will activate the Truth of Formations, which will give a bonus to the pillar (eg. Mind of the Sword, with 4 Jian activated gives Artifact Mastery + 10)
Third, there's the auxiliary rate, if the elements are feeding each other the overall bonus from that auxiliary node will be multiplied.