You would have to hunt down which font is applied to the incoming damage text rather than the outgoing text.
What that addon does is a quick and dirty way of changing the fonts to another font. However, the same fonts are used in other elements of the game so sometimes it will affect those too.
I'm not a font expert but while working with my own font changes in my own addons I have noticed there are 2 different ways of dealing with unique font changes.
SetFontObject - This will set a specific font already set up as useable by that text object.
GetFontObject - This will get the font used by that text object which you can change but will affect that object wherever else it is used in game.
SetFont - This will allow you to change the file, size and/or style of any text object without affecting the fontobject assuming you are attaching it to the text object itself and not the fontobject.
GetFont - This will extract the font details of that text object as 3 individual elements which can then override without affecting other objects using the same font object.
Without writing your own addon I would suggest you ask the creator of that one to investigate the problems you have come across and try and rectify them and also to locate and reflect the incoming damage text font too.
This is the based Font Object for the combat text. You could try using this to override all CombatTextFont references which hopefully are not used elsewhere. An example below:
Code:
<FontString name="CombatTextTemplate" inherits="CombatTextFont" hidden="true" virtual="true">
These are the individual CombatText font strings used in Blizzard_CombatText.xml/lua the default combat text addon.
Code:
<FontString name="CombatText1" inherits="CombatTextTemplate"/>
<FontString name="CombatText2" inherits="CombatTextTemplate"/>
<FontString name="CombatText3" inherits="CombatTextTemplate"/>
<FontString name="CombatText4" inherits="CombatTextTemplate"/>
<FontString name="CombatText5" inherits="CombatTextTemplate"/>
<FontString name="CombatText6" inherits="CombatTextTemplate"/>
<FontString name="CombatText7" inherits="CombatTextTemplate"/>
<FontString name="CombatText8" inherits="CombatTextTemplate"/>
<FontString name="CombatText9" inherits="CombatTextTemplate"/>
<FontString name="CombatText10" inherits="CombatTextTemplate"/>
<FontString name="CombatText11" inherits="CombatTextTemplate"/>
<FontString name="CombatText12" inherits="CombatTextTemplate"/>
<FontString name="CombatText13" inherits="CombatTextTemplate"/>
<FontString name="CombatText14" inherits="CombatTextTemplate"/>
<FontString name="CombatText15" inherits="CombatTextTemplate"/>
<FontString name="CombatText16" inherits="CombatTextTemplate"/>
<FontString name="CombatText17" inherits="CombatTextTemplate"/>
<FontString name="CombatText18" inherits="CombatTextTemplate"/>
<FontString name="CombatText19" inherits="CombatTextTemplate"/>
<FontString name="CombatText20" inherits="CombatTextTemplate"/>
Example: Assuming that you have no other combat text display addons and have the default blizzard combat text messages displayed I would hazard a guess that this set up during the ADDON_LOADED event may be all you need to do.
Code:
local fontObject = CombatText1:GetFontObject();
fontObject:SetFont(fontFile,fontSize,fontStyle);
Simply replacing the values in SetFont() with your chosen file, size and style ( "OUTLINE", "THICKOUTLINE", nothing) and because they are all based on the same font object just changing the first combattext line should be enough to update the rest. I haven't been able to try this out as of yet due to server down time but if that doesn't work try replacing CombatText1 with CombatTextTemplate or CombatText and see if those work.