[Phanx]

The simplest option is probably just to do this:

hooksecurefunc("GuildRosterButton_SetStringText", function(buttonString, text, isOnline, class)
     if strmatch(text, "%D") then
        -- text contains something that isn't a number, use the standard font:
        buttonString:SetFontObject(GameFontHighlightSmall)
    else
        -- text contains only numbers, use a smaller font:
        buttonString:SetFontObject(GameFontWhiteTiny)
    end
end)

If there are some other columns that display numbers that you don't want to make smaller, this might work, but I didn't test it:

hooksecurefunc("GuildRoster_Update", function()
	-- What viewing mode are we in?
	local currentGuildView = UIDropDownMenu_GetSelectedValue(GuildRosterViewDropdown)
	
	-- In most viewing modes the target column shows a level:
	local fontObject = GameFontWhiteTiny
	-- But in one viewing mode it shows text:
	if currentGuildView == "guildStatus" then
		fontObject = GameFontHighlightSmall
	end

	-- Loop over all the rows in the roster list:
	local buttons = GuildRosterContainer.buttons
	for i = 1, #buttons do
		local button = buttons[i]
		-- And apply the desired font object:
		button.string1:SetFontObject(GameFontWhiteTiny)
	end
end)