I'll take Decados.
I think engineers are expensive enough. The problem is that it becomes virtually impossible to siege another world if you don't have decent fleets to ferry troops in. And you won't have those if you don't have the resources to create them, resources generated by manufacturing cities. It would take an entire re-balancing of the units to change this. I've messed around with really trying to get a great re-balancing of EFS (while maintaining the interesting direct vs armor and close vs infantry dynamic Nova got rid of), but eventually gave up due to the lack of general interest in the game when I was doing it. Hyperion uses various formulas to determine unit "value", and Nova is allegedly balanced, but I have my reservations about whether this is actually accomplished in practice.
I like plague on. One of the rules we'll be following is to not intentionally starve your cities. But I'd be ok with allowing even that. EFS really starts to see unit creep, where you have a ton of units on every world that make invasions prohibitively difficult. Starvation and plague leads to destruction. Destruction is good.
As for rebellion, I'm not sure. On random worlds, rebel units are at 0% loyalty. This means any you capture will rebel against you. I don't think it's a big deal either way.
One possible solution to mainiac's dilemma is having us all skip our first turn. We could all agree to take absolutely zero action on any screen, for any reason. I don't see any harm in this and it would allow him to show us a bunch of maps for us to decide on.