Can a weak impale replace a stronger impale ?

Lets say an enemy has max impales from my main skill (Ballista Totem) then i hit him with a secondary skill (like a curse on hit setup).
Will the weaker impale coming from the secondary skill replace one of the main skill's impales ?

Eh, no. Number of Impale stacks on target technically isn't limited and every Impale stack individually snapshots the damage of hit that applied it. The limit comes from the fact that each hit also expends one charge from every Impale stack present.

There is no limit to impale as TN_Sylar said. You impale an enemy, the next 5 times the target is hit will consume 1 impale. Hits can consume an unlimited amount of impales, and anyone can consume.

If you are in party play and 3 other people impale the enemy and then you hit, you trigger 1 of each their stacks of impale. If you hit the enemy 5x and each hit impales,then your next hit will have 5 impales.