Okay.

Halfback is the guy who takes the majority of the running plays. He can also be thrown to and used as a receiver.
Needed: Strength, Speed, Carrying and possibly Catching.

Fullback is the guy who will block for the QB on passing plays. He will also block for the Halfback on running plays and will be expected to run with the ball for a few plays.
Needed: Blocking, Strength, Speed, Carrying

Strong Safety lines up on the strong side of the play. This means that he will be on the side of the opponents Tight End. Basically he assists in stopping the run, so he will be closer to the line that the Free Safety.
Needed: Vision, Intelligence, Strength, Tacking, Positioning

Free Safety lines up alot further back in the backfield. His job is to help the cornerbacks with the receivers and pick off the long-ball threat. Additionally, he may be used to cover the slot receiver.
Needed: Vision, Intelligence, Speed, Tacking, Positioning

This is just my opinion, I'm sure many will have a different view on it

Yeah.

But obviously it depends on the opposition in the NFL. For example: If it looks like the team will run the ball, the Free Safety will move up more towards the line.

So in GI, if a player has a terrible QB, you can predict that they will run the ball more often than pass. So, you would have two Strong Safeties instead

Yet

So what? You can only play 2 of each

All I'm saying is, the game will become more pass-happy soon (I hope!). It's just a safer bet to run the ball right now I guess.

I meant it doesn't matter because you can only play 2 of each on this game. Not in NFl...