"Medium" or "Middleweight" is a classification of gladiators who are usually balanced in terms of attributes and have a wide variety of skills for many situations. The Medium gladiator classes consist of the Barbarian, Legionnaire, Murmillo, Undead Legionnaire, and the heroes Ludo, Urlan, Ursula and Valens. Dark Legionnaires and Undead Melees fight as Medium gladiators but cannot be recruited directly into your school.
Since the majority of the game's heroes are Medium gladiators, there are not too many other Medium classes to choose from. Medium gladiators, including the game's heroes, are best used against Lightweight opponents and should not be used against Heavyweights. Legionnaires and Murmillos are also effective against Support classes since they balance Power, Defense, Movement and Constitution unlike any Light or Heavy class can.
Barbarians are more powerful than other Medium classes but have lower Defense, and cannot equip a shield. A Barbarian's best point of strength is in their ability to shape-shift, regenerating much of their lost HP in the process. Undead Legionnaires can be recruited from tombstones after acquiring the Talisman of Unlife and have many unique Innate and Attack skills, but are otherwise not very different from Legionnaires.
As for the heroes, Valens features many useful Innate skills and is a very well-rounded fighter, while Ludo more closely resembles a Legionnaire in both attributes and skills. Ursula has some unique attacks that will affect a large area around her. Urlan is more or less a typical Barbarian, with a few additional skills such as the ability to shape-shift into a Plains Cat.
Medium gladiators will have a fairly large damage boost when attacking Light gladiators, but no damage bonus or penalty against Heavies or other Medium warriors. They also take more damage from attacks made by Heavy gladiators. The relatively high Accuracy of Mediums, on top of the damage bonus, make them an excellent choice for fighting one-on-one with Lightweights, though many Medium classes (usually with the exception of Valens) do not have a practical answer against Heavies.
Since you will have 3 Medium gladiators in your school by the end of the game anyways (Valens, Ursula, and Urlan) you do not necessarily need to recruit another one if you want the best diversity for your school. If you do choose to have a fourth, the best choices are generally the Murmillo or Undead Legionnaire.
Bears, Scarabs, and Scorpions are classified as Beasts and can be used to meet league requirements for Beasts, however they receive a damage bonus against Light classes (including Plains Cats and Wolves) and take more damage from Heavies (including Greater beasts). It is important to understand that Bears do in fact fight as Mediums and not Heavies, as a tip from Usus in-game falsely states.