A full team of six anywhere close to early on is a bad idea for a bunch of reasons, primarily narrative reasons.
For one; the thread's already struggling trying to spend any time with the human characters in the story. Arvis has repeatedly emphasized through the story that Mary's kind of doing a bad job because she's ignoring most of Class VII when it comes to actually making friends; she effectively only spends time with Ostia, Nemona and Erika of her own volition, and otherwise only spends time with people like Elise, Theodora and Atticus when Arvis mandates it via associating those characters with actions the thread wants to take independent of who's with them.
The more Pokemon Mariana has, the more of a problem this will become, because it adds extra characters who need characterization to the roster. Those are characters who effectively always have the potential to be on-screen, and who are a major part of the draw of a Pokemon fanfic to a lot of readers.
This is something that's less of a problem later on when all the characters are firmly established later on. When everyone in Class VII has had their time to shine and we're deep in their respective plots and they're fixed in the reader's mind, it's fine to have a wider cast. But trying to introduce too many Pokemon early on is a really bad idea for this reason.
Second;
The quest is designed to take place over a long period of time; not just one year, but multiple years.
Catching most of the Pokemon for Mary's team too early on means that there's relatively little options for progression later on down the line beyond just developing new moves. Evolution isn't designed to happen months and months and months down the track in the quest's mechanics; it's designed to be something that happens within a couple of months of the capture max.
If captures are spread out over a significant length of time- say, 2-3 years- then there's a relatively steady rate of progression. Mary can capture a fourth Pokemon around month 5 or 6, then a fifth around year's end, then a sixth a few months later or whatever; this gives the fic a constant sense of Mary's continued growth.
Option the last...
It also just makes battle scenes more and more of a headache for Arvis to write?
If Mary has six Pokemon, then the Gym Leader will have to pull out six Pokemon in turn to face her. By the point Mary has six Pokemon, this will mean that every Gym Battle will feature nearly triple the Pokemon that the current battle did. This gym battle was already really fucking long; triple the Pokemon will mean that Arvis will be staring down the barrel of 20k+ word updates for every gym battle around that point. It'll also mean that every battle Mary gets involved in will have to accommodate for the sheer number of Pokemon she has, there'll need to be continual wordcount devoted to Mary training six or more Pokemon up over the course of a year's worth of updates, etc.
It's better to shoot for spreading capturing a full team of six out over a long period. It's just a good narrative choice that will allow Arvis to actually keep writing updates that don't take a month each to write.
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