If I remember right ... Gus & Neo are both males, right?
Are there any female chinchillas in your home? If there are, I would not put two males together in the same cage.
Often, the original chin can sometimes be jealous of the new chin and the attention paid to it by their human parent(s). This does not seem to be the case since Neo, the newest one, is the most aggressive.
BUT, Neo could be naturally the jealous type and be jealous of Gus and your affection for him. If this is the case, you might try always taking care of Neo first and last (showing him the most attention) AND ... should Neo attack Gus, then Neo would be put in a time out cage (usually very small) and left where he could see you play with Gus.
Chinchillas are smart, you might be surprised how quickly they learn that the one that starts the fight ends up in "time out" and being ignored for a period of time.Chinchillas have all the emotions humans have. Each has it's own personality, and, as often seen with humans, some personalities do conflict. Haven't you met some one and instantly seemed to dislike them? That's due to a conflict of personalities, usually. The same can be true with chinchillas. It is rare, but does happen.
You also have to keep in mind the difference between playing, rough playing, scuffling, dominance 'play', and actual fighting.
They can seem very similar, if you are not accustom to seeing all of these.
Playing is usually bouncing back and forth and a little chasing each other.
Rough playing is where one is being a little to rough or rougher than the other ... this will sometimes escalate into actual fighting OR the retreat of one from the other, due to fear, or may end up in a lifelong friendship.
Scuffling is playing + a bit of grabbing and going into a roll together, there may be a little fur flying, but there is no biting or blood. As long as this is as far as it goes, they will be OK, but keep an eye on them for a while.
Dominance play is deciding who will be the alpha chin ... humping in this instance is not sexual, but is showing dominance. This usually plays out in one being the alpha chin and the other being the follower, but remaining friends.
BUT ... When two very strong, dominant chins are put together, the dominant 'play' can end up in a dangerous fight that can mame and/or kill one or both.
OR ... If a strong, dominant bully is put with a shy chin, it can, sometimes result in the bully not allowing the shy one to eat or drink and the bully will often beat up on the shy one just because he can.
Dominance play has to be watched carefully and followed for a long time.Fighting is (can be) immediate fighting, or a combination of any and all of the above, but ends up in large amounts of fur flying, biting and blood. If I ever have one draw blood from another, they are separated permanently. I do not take the chance of it happening again ... but, this is what I do ... you have to make those decisions for your own chin, but be willing to accept the result, the responsibility, the vet bills and the possible death of one or both.
Chinchillas stress easily, when they stress is when actual physical/medical problems can develop that would not do so otherwise. For this reason, I do not try to force any two chins together, if is seems like one or both do not want to be with the other one.
Chinchillas have allergies to
perfumes, colognes, candles, room sprays, bath/baby powder, cleaning products that often affects the lungs of a chinchilla ...
all of these contain artifical/man-made odors from chemicals ... I would not use any of these to introduce one chin to another.
I have found, allowing both to use the same dust bath and share the same toys or swap cages, will often work well.
Chinchillas do often act different when their human parents are not in sight. It is best to allow them time in the room alone together, but be right outside the room to come back in quickly, should something sound 'out of the norm'.
The chinchillas will decide what they want ... but, how far you let it go or get, is up to you.
Keep us posted on how things are going!
Jo Ann