Here's line 20 from AceConfigDialog-3.0.lua:
Code:
local gui = LibStub("AceGUI-3.0")
Maybe all you need is an instance of "AceGUI-3.0", which "FishingAce" has in its' "Libs" folder, but TMM does not. I'm guessing that's why there's an error for it when run alone but not when run in conjunction with "FishingAce".
You could probably copy "AceGUI-3.0" from the FishingAce "Libs" folder and paste it into your TMM "Libs" folder and fix the error when not running any other addons. Although, and I'm not sure about this, you might have to edit the TMM "embeds.xml" file (to add the new lib to it) to make it work right.
Of course, I could always be wrong, and I'm sure someone will step in and correct me if I am.