Very interesting to read more about gerbil dynamics. Do they not hurt each other like hamsters do when kept together and its more a juvenile "play fighting" behaviour?
It is juvenile play fighting although when it comes to "play fighting" I always like to draw the comparison to children playing role playing games like pretend cooking or looking after dolls. They're playing but they're also practicing skills for later in life and usually the baby gerbils that regularly come out top in play fights will be more dominant as adults. They themselves also don't see it as playing at this age. They take themselves very seriously!
They don't hurt each other because gerbils only bite hard enough to cause damage when they're being properly aggressive, which pups of this age don't have the capacity to be (unless extremely mistreated). The bouncing around/boxing/wrestling looks violent but they're pretty tough and I've never heard of an injury from this type of playing.
Hamsters start out play fighting just like this but then starting from about 6 weeks they tend to (always in the case of Syrians, and very often for dwarfs) start to develop an individual identity and want to live alone and the fighting becomes more real and more serious. It takes an experienced breeder to know the difference with hamsters so they can separate them in time. Conflict can also build up gradually over time which is why people often bring home a pair of dwarfs, and everything seems fine at first but eventually it all goes wrong.
Because gerbils are social, instead of developing an individual identity from 6-8 weeks of age, they start to develop a group (or clan) identity. Before that age, they don't have a sense of themselves as a clan. You could merge two groups of 4-week-old gerbils and they wouldn't care at all because they're not territorial yet. By the time they are 8 weeks old, they will be starting to see themselves as an established clan with its own territory to defend and they'll be resistant to accepting any new gerbils.
Removing any gerbils from a clan of 3 or more can also be risky as you'll be interfering with established power structures and hierarchies (e.g if you remove the dominant gerbil, you'll leave a power vacuum which may cause fighting), therefore, for pet-keeping purposes, breeders will separate gerbils into the pairs or small groups that are intended to stay together for life by 8 weeks old, before the hierarchies and power structures are too well-established.
Gerbils also prefer to live in pairs rather than bigger groups, and a bigger group may start to gradually push out some members beginning from about 8 weeks, but in practice this tends to happen much later when they're a few months old.
Attempting to introduce two pairs of adult gerbils (which a lot of people unfortunately try to do) would essentially have the same effect as trying to introduce two adult hamsters. Both gerbil pairs have a clan identity that excludes other gerbils, and each hamster has an individual identity which excludes other hamsters. And both will almost certainly end up fighting, sooner or later.