So I just recently found Mongoose (it's the only version I own, sorry)
I'll assume then that you do not have access to
LBB4 Mercenary which would provide some of the necessary nuances you're looking for here, so I'll try to provide one perspective that you can make use of.
My original headcanon of having them wear the cheaper, less capable armor while in their first term as a way of weeding out those who would not make the grade is wasteful of the trememdous resources the Imperium is already investing in these people. Making the BD standard issue for all marines, at a cost increase per marine of only 200,000 Cr, gives a very significant upgrade in terms of survivability (Armor 12->25) as well as opening the field tremendously because the physical stats needed are much lower, as the BD is powered and therefore provides significant assistance, carrying capacity-wise.
In LBB4 extended character generation for Army and Marines, the ONLY opportunity to get Battle dress skill is from the Military Occupational Specialty (MOS) Table, which you roll on ONCE for your first year of Basic and Advanced Training before rolling for unit assignments the following year. Here is what that table looks like:
What this means is that Army can have a choice of Arm from Artillery, Cavalry, Infantry or Support after being recruited.
Marines have a choice of Arm between Marine or Support.
Commando MOS requires attendance of Special Assignment: Commando School ... after which, when re-enlisting for another term, you are eligible to change arms to the Commando Arm at the beginning of your new term, making you eligible for a year of Advanced Training and a fresh roll on the MOS Table.
The MOS Table is the
only place in extended character generation in CT that Battle dress skill can be obtained from ... and the tech level of the recruit's world needs to be TL=12+ in order to have
a 1 in 6 chance of learning Battle dress skill.
Point being that Battle dress is not "universal" for marines, according to the extended character generation system. Most marines will not have Battle dress skill ... and the most Battle dress skill that can ever be learned is Battle dress-2 from being a Marine MOS first and then becoming a Commando MOS later, giving you 2 rolls on the MOS skills table and you roll 6 both times. In other words, Battle dress skill is
such an exclusive club that most marines never get the skill and only some army commandos ever learn the skill ... and it is extremely unlikely a character will ever learn Battle dress-2 skill (let alone 3+).
The only other path to learning Battle dress skill is via Special Assignment: Cross Training, which is only available to Enlisted and NCO ranks, not Officers. Army cannot cross train as marines or commandos. Marines cannot cross train as artillery, cavalry, infantry or commandos. So the only viable path to learning additional skill levels in Battle dress is to be a marine in the Support Arm, receive cross training as a Marine and roll 6+1: Battle dress from a year of cross training. Only the Marine Support Arm would be eligible for this result from repeated cross training assignments.
So in terms of the marine unit you're featuring in your story, there are a few possibilities why Combat Armor instead of Battle dress is being issued to the troops featured.
- Unit specialization: Flower's unit is primarily a(n armed) recon unit, rather than a direct combat unit, so they get Combat Armor instead
- Tech level: Flower's unit is home based at a world with TL=11- and thus doesn't offer training in Battle dress.
- Luck of the draw: Flower didn't "make the cut" during Basic for diversion into the Battle dress program for his Advanced Training.
I can also imagine that the logistics tail for Battle dress is going to be "heavier" due to the high technology than the alternative of Combat Armor, just like how "cavalry armor" can be a "heavier" logistical lift than "mechanized infantry" (let alone dismounted infantry) that can be more dependent upon supply chains to keep operational. So you wind up with a case of Battle dress being the "tip of the spear" while Combat Armor is the "blade of the spear" ... if that analogy helps any.
Battle dress is therefore reserved for what amount to specialty shock troops, while everyone else gets Combat Armor.
For the purposes of your story then, Flower and his unit just aren't one of the specialty shock troop groups, so they get Combat Armor instead.