I don't know of anywhere where the timeline has been established, and I suspect it runs more on Comic Book Time.
Also, Bubbles wasn't born as a combat unit. She was a volunteer that wanted to serve, suggesting that she had a life before that, and presumably a different chassis.
The timeline was last updated in November of 2015.
You make a good point, that Bubbles could have had formative experiences before receiving a humanoid body. But it also sounds like she has had that body for quite some time, in-strip. And it is a very convincing human-looking body aside from the coloration and being a muscular giant, both of which were intentional and I assume not the only type being made. Yeah, continuity kind of suffers there.
On the other hand, since she was part of a military test project to see how AnthroPC would do in the military, it's not very unlikely that they also kept modding or changing out her chassis during the entire time, both as technology evolved and as things came up through testing.
"Bigger body is more imposing, can carry more, and is still no more suspectible to being hit since an arm wound doesn't take out the entire soldier as on a human" - new chassis that is as big as transport vehicles can handle.
"Integral armor gets more expensive or time-consuming to fix, and can be occasionally too bulky" - rebuild, more motile torso and external armor plates
"Some fellow troopers have mentioned trust issues or cooperation problems because harder to read AnthroPC" - new cutting edge head with more emotive ability.
Etc. The point being that just because she probably did at least one year in service, she doesn't have to have started out looking like she did when mustering out.