Good questions and answers! I have a vested interested having been in Tomorrow for a year now and at least another year or possibly more before I decide to graduate and have been labouring about the best defence combination to use as DAs in GvG.
In regards to attack, against a single wave defence, probably 1 stealth and 7 rogues when one is playing fast on auto, as in GvG. If one is playing slow, maybe one champ and 7 rogues, setting the champ aside. I may be splitting hairs as the champ has a little extra attack and defence but if you are tucking it away on the first round you are effectively giving away one shot.
Given the stealth aspect to the tank, you can use quite a few rogues with a small number of tanks in DA and don't run a high risk of having your tanks defeated first, as the opponent attacking will likely get to your rogues before they get to your tank. The exception could be the ultra AP given it lands anywhere and has the 'contact' features which means that if the opposing stealth tank hits it, the AP can still strike the tank even though it is hidden in a distance.
For me anyway, the main question is whether to choose 1 stealth with 7 rogues, or 2 stealth with 6 rogues, for defence.