What type of cardio should I be doing?
This depends on the position (and the athlete). The needs of the pitcher are different than the needs of the position player. Pitchers are making one movement in the same direction while position players run from base to base, chasing down fly balls, move laterally for ground balls, etc. This requires that their training to be different. Pitchers do not need to focus their program on sprint work, change of direction/lateral movement as much a position player, but rather on developing speed-strength. The following article Velocity Development Program: Speed and Agilitysheds more light on this topic. It is also further discussed in DrivelinePLUS video Differences and Similarities when Programming Lifting for Hitters and Pitchers.