I mean persoanlly id say focus mods mainly on GAC, as that is the main crystal income, and dont worry too much on conquest and TB. Conquest is more about datadisks then mods as all enemeys have 200+ speed added to them. For TB i only remod for KAM, otherwise once you have solid loadout setup from GAC i find most TBs arent that bad, i usually get 3-4/4 on most battles unless im trying a random new team (like off beat omi ROLO teams) or trying to use my weak jedi. (I have enoigh jedi for 1 mission but the other never goes well for now).
Challenge rancor id say depends on if you really need the higher ranks in it, or have to get so much damage to help clear. Otherwise if your in agood position i wouldnt worry about mod optimization.
As for team needing special care, id say that mainly boils to the normal teams that prefer specific turn orders with speed. Drevan, Imp troopers, BAndo, normal mando, ect.
Best advice for trying a GAC focus mod loadout primarily is start with the important characters and teams first and make them the best (super fast Badstilla, healthy Grievous and Padme, Imp trooper perfect turn order, or just whoever are your strongest teams)