As shown here Crusader, Aventine and Foundry have several 'standing at ease' poses in their ranges. Size-wise I would go for the Steve Saleh-sculpts from Foundry, for they're matching the heighth of most current 28mm gladiator models.
That said, it's rather dubious if actual soldiers were employed as guards in an arena. I know that's the common image, but there might have been rarely a need for guard duty at the games. After all gladiators were (almost) exclusively professionals with little interest in running away. If convicts were sent in, members of the gladiator schools could double-function as guards (and, of course, executioners) as well. In addition, most likely, as a private business every ludus had its staff of trainers, bouncers and the owner's bodyguards. These might have been either hired ex-soldiers or former gladiators who, upon their manumission, had become members of the lanista's household/patronage. [Here's some additional thought on that over at
Roman Army Talk.]
Therefore, if you have to have guards, go for un- or lightly armoured types. Even soldiers had some kind of undress uniform, which was obviously worn on most occasions and consisted of just their military belts and sidearms. Or perhaps you could provide the models with gladiatorial equipment or outdated military items.
A final note: Most figure ranges are based on pictorial evidence from the late 2nd to 5th century AD. Therefore Middle Imperial or Late Roman ranges might be an unusual, yet "historically accurate" choice if you really want military-style guards.
