A Tactical in a cruiser will have some extra damage vs extra survivability than an engineer would provide. Mainly from the abilities you pick up as you level for space.
You can mix and match the player class and the ship fine though; in OB I flew all 3 ship types with my engineer. And in live I'll just stick to science and cruiser. I've found as an engineer, my engi/science vessel seems to have a little more survivability than my friend who flies sci/sci.
As for the code, check the main page for the game, there's a "Enter Code" button on the middle right.
The difference between Tac/Cruiser and Eng/Cruiser isnt much its just means that if your a tactical captain you get abilities like Attack move Alpha which gives you boosted crit etc whilst a Eng captain will get different skills all it really means is a tac captain will do a little more damage then a eng i think
Indeed.. at level 11 after you spend your points you get a request to see the admiral back at starbase 1 to get your promotion...
you're given a ship token good for *ONE* ship available at your current rank which will be Lt. Commander 1. Go to requisitions and speak to the person mentioned in the quest dialogue and simply select cruiser. Just like the Miranda class you start with you'll have three options for configuring the look of your cruiser.
you will have a new bridge option though, one that resembles the TNG bridge, it costs about 1k eCredits but looks really good (though the scaling is slightly big).
I played a tactical officer in a cruiser in beta, and are now playing a engineer in retail with a cruiser, and imo there isn't a huge amount of difference between the two, though I do prefer brace for impact and the damage buff over the engineer abilities.
Later tiers it maybe more beneficial to take a engineer due to the huge hulls and shields, rotate shield frequencies can regen alot of shield health at those levels which is what I believe is nessessary, but also in the higher tiers the cruisers split off into tank and dps so either shall do, it just means you will have to take atleast a minimial one or two engineering abilities which regen shield/repair if you go tactical.