The "All Marines are BD Troopers" argument got started during TNE, when Striker II came out, and showed ALL imperial troops in TL14 LBD or TL15 HBD, if they weren't in tanks. This was based off a little read article by LKW, which said the marines were entirely infantry, and all BD.
Which was really a way of saying "They're tougher than SW Imperial Stormtroopers...."
I'd never even heard t=oif that (IMO LUdicrous) article until well into the TNE era.
Since Loren wrote GT, he made all GT IM's have battledress skill at 1/2 point. So in GT, yes, all marines are at least trained in BD.
In CT/MT, aside from loren's article, there is little conclusive evidence of extensive BD training, and great argguments against it from a rules poit of view: BD skill is hard to get under all forms of CT/MT CG.
Under TNE, those are specifically NOT Imperial Marines, but RCES Marines.
Hopefully, T5 will revise the setting as well... Traveller is groaning under its own weight of internal inconsistancies and bad decisions along the way... (Active readers will know many of my complaints...)
And the T5 draft doesn't mandate Battle Dress as a marine default skill... (Nya! at Doug....)
T20 presented a non-BD marine, but made so that marines could be an all BD marine force...
I assume 8 man squads in Traveller: 2 striker bases! (and the CA/BD carry load of an Astrin APC)
Since most modern forces are using 3 or 4 man fire teams (more often 4) as a subfivision of the squad, I think squads will typically be 8-13 men, although for BD elites, I've allowed as low as 7 man squads: SL, 2x 3-man FT's,
Certain line army unite IMTU are using 20man squads: SL, ASL, Comm, Medic, 4x 4man FT.
I assume 10 man GrMechInf squads: 2 FT, Driver, Gunner.
IMTU, the IM's are very flexible, and operate on the "mixed forces regiment" model. I'ts not uncommon to see a regiment with its own arty, either! Compined FGMP and Back Rack Mortar units can POUR ON THE HURT.
Then again, I released upon an unsuspectin TML a Meson Beam euipped BD suit for TNE...