The number of survivors really depends on the strength of the cybrids - weaker cybrid teams (for example, if they started out with only one convert and it was unreliable) are generally balance by taking members from both the Imperials and Rebels and making them neutrals. If I have more than one neutral, one (and maybe both) non-cybrid sides will have a persuader. There are really two options here:
Pull survivors from (or mostly from) one side: Give that side a persuader, but not the other. Example 5I-2N-3R-1C
This leaves the rebels a bit weaker earlier off, and losing their persuader is a huge hit, but since they can probably convert 2 or 3 of the Imperials as well, leaves them in a potentially strong position late game. Since a survivors job is merely to end the game as quickly as possible, this DOES give the imperials a decent advantage early on - but then, it only takes one rebel persuade to achieve parity. This would probably be somewhat balanced by giving the teams an equal number of power roles (so the rebel team would probably end up doubled at least once, and then have the ability to get more power roles as well, since I generally favour neutrals for roles like Doctor). Note that this can be done with a stronger cybrid team since it still leaves a decently powerful "town" block.
OR
Pull survivors equally from both teams. For example, 2I-2R-1C-6N
This is a bit like a three way cybrid-team game in many ways - all teams would have powerful leaders and the ability to recruit. This make bussing, rather than defending, a VERY difficult choice if they are the persuader. Its risky - and it means a lot less direct opposition to the cybrid early on. The neturals know they are going to be recruited by someone and probably won't have much incentive to be active - which is why I'd probably add a few flavour mechanics on top of this sort of game (more than in most) to keep things moving.
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The first option up there is by and far my preferred setup, though the others can be fun.