I don't see Starscream picking anyone to be a second in command and I don't see anyone following him for more than a few plots. Starscream (or at least his cartoon personality) wasn't much of a leader or a thinker and had no real rapport with anyone else. Didn't Thundercracker and Skywarp hate him too?
Starscream wouldn't have had a real second in command, only dumb muscle who was easily satisfied and wouldn't care or be smart enough to oppose him. And that definitely wouldn't be bruticus. The combaticons hate him, and bruticus had his programming altered to obey megatron and not starscream. Really, none of the gestalts would work, because they all either hate him, think he's an idiot, or would try to pull something to take over for themselves(and I'm sure starscream would be aware of this). Maybe if he could control abominus, but that's a longshot. Soundwave is a bad choice, because given the chance he'd find some way to bring megatron back, or investigate and found out that galvatron was megatron and follow him instead. Shockwave wouldn't work, because shockwave would think starscream is a complete idiot. Starscream would have been screwed as leader of the decepticons no matter what because anyone who doesn't hate him, would rather just overthrow him anyway because they have no loyalty. The decepticons would have fallen apart fairly quickly, and be broken up into smaller groups fighting with eachother for total control(which has happened in the comics).
I really like the idea of splinter groups forming up within the Decepticon and Autobot armies and fighting amongst themselves like in the WW series (whichever one had that). I think in the end though that any team being lead by Starscream would fall apart because he seemed like he would care less about his troops than Megatron did.
I don't think he would have a second in command. I think a better question is, who would be the new Starscream and give it to him like he did Megatron?
I don't think it's a question of which Decepticon does it because they'd all gang up on him and kick his ass. I think any number of Cons would want to just murder him after a matter of weeks. Which Decepticon do everyone think should have won during the brawl on the shuttle. I know they described a scene were Starscream ambushed the victor so he could win, but who was that one victor supposed to be?