[Rainrider]

You both missed the last argument of the ClassIcons' PostUpdate function, which tells you whether the number of bars changed. Here is what I use for reference:

Register the element:

local AddClassPowerIcons = function(self, width, height, spacing)
    self.ClassIcons = {}
 
    self.ClassIcons.width  = width
    self.ClassIcons.height = height
    self.ClassIcons.spacing = spacing
 
    local maxPower = 5
 
    for i = 1, maxPower do
        self.ClassIcons[i] = self.Overlay:CreateTexture("oUF_Rain_ComboPoint_"..i, "OVERLAY")
        self.ClassIcons[i]:SetSize((width - spacing * (maxPower - 1)) / maxPower, height)
        self.ClassIcons[i]:SetPoint("BOTTOMLEFT", self.Overlay, (i - 1) * self.ClassIcons[i]:GetWidth() + i * spacing, 1)
        self.ClassIcons[i]:SetTexture(ns.media.TEXTURE)
    end
 
    self.ClassIcons.PostUpdate = ns.PostUpdateClassPowerIcons
end
ns.AddClassPowerIcons = AddClassPowerIcons

The PostUpdate function:

local PostUpdateClassPowerIcons = function(element, power, maxPower, maxPowerChanged)
    if (not maxPowerChanged) then return end
 
    local self = element.__owner
    local width = element.width
    local height = element.height
    local spacing = element.spacing
 
    for i = 1, maxPower do
        element[i]:SetSize((width - spacing * (maxPower - 1)) / maxPower, height)
        element[i]:SetPoint("BOTTOMLEFT", self.Overlay, (i - 1) * element[i]:GetWidth() + i * spacing, 1)
    end
end
ns.PostUpdateClassPowerIcons = PostUpdateClassPowerIcons

Add the element to the frame appropriate unit frames:

if (playerClass == "DEATHKNIGHT") then
    ns.AddRuneBar(self, 215, 5, 1)
    ns.AddTotems(self, 60, 5)
elseif (playerClass == "DRUID") then
    ns.AddEclipseBar(self, 230, 7)
    ns.AddTotems(self, 30, 5)
elseif (playerClass == "HUNTER") then
    ns.AddFocusHelper(self)
elseif (playerClass == "MONK") then
    ns.AddClassPowerIcons(self, 215, 5, 1)
elseif (playerClass == "PALADIN") then
    ns.AddClassPowerIcons(self, 215, 5, 1)
elseif (playerClass == "PRIEST") then
    ns.AddClassPowerIcons(self, 215, 5, 1)
elseif (playerClass == "SHAMAN") then
    ns.AddTotems(self, nil, 5)
elseif (playerClass == "WARLOCK") then
    ns.AddWarlockPowerBar(self, 215, 5, 1)
end

Apart from that, adding combo points only for druid and rogue is a bad idea, as other classes can use them too when in vehicle (Aces High! comes to mind)

EDIT: Ouch, Phanx got it right of course, I kind of have the annoying habit of reading too fast and thinking to slow. I don't think the call to ForceUpdate is needed, as oUF does this for you when it sets up the unit frame. But I would trust Phanx more than me, so you'd better check.