Which turret is best to put out on the field? And where?
A strategy that's worked well for us has been starting out with laser turrets at first (when there's little money to go around) then replacing them with rocket turrets as they get destroyed. I usually put a rocket turret at each side entrance and an ice turret in the middle of the front archway to slow down the main group of bots while all the turrets around them rip into them. A longshot in each of the back corners works pretty well too.
In The Scramble I always try to build rocket turrets since the bots seem to prefer attacking turrets before the moneyball, and since they have more armor than laser turrets I can get over there while they're clumped up and drop a smoke bomb (assassin) or air strike (support).
What classes would be best to choose? What skills to upgrade?
In Blitz, we've settled on one support, one gunner, one tank, and either an assault or an assassin.
In The Scramble, a support on the upper level continually healing a tank who's just spamming close-range area-effect skills works wonders. Park the tank near the front archway in the middle of all the turrets for maximum carnage. The assassin can also run around cloaked and drop a smoke bomb on a cluster of gremlins to freeze them, making them easy to take out before cloaking again.
As far as what skills to upgrade, just upgrade whatever skills you find yourself using most. Eventually you'll have them all fully-upgraded anyway. As an assassin I focus on the cloak skill and the passive (bottom) skill to upgrade to the katana. As support I focus on the passive skill first so I can heal faster, then focus on upgrading my mini-turret, then on hacking.
How many players is optimal?
After some experimentation, I'd say three. We finally beat The Scramble when we started out with four, then lost a player around round 8. By then we had almost every turret up the last two rounds at least seemed much easier :)