I highly doubt a legionary in good standing, wearing his legionary gear, would have fought in the arena. Gladiators were not considered "respectable" (despite highly successful ones becoming "rock stars" in the Empire) and I strongly doubt a legionary would be allowed to degrade himself and the arms of Rome by appearing in the arena. Gladiators with specific types of kit were paired-up with certain other types for the sake of a good show. For example, the murmillo usually fought the thraex or hoplomachus (all three types being comparatively heavily armored, with full helmets, arm guards, one or two greaves, large shield, and short sword; there are some indications that on occasion he fought the retarius. The retarius usually fought the secutor