Questions about Seleukid units.

[QwertyMIDX]

It's actually more a propensity of modern historians. The roman authors usually say "armed in the roman manner" which we read as 'imitation legionaries' but seems to refer to, basically as you said, fairly well armored troops (most often mail), with large shields, who didn't fight in a phalanx (with the caveat that this almost always done with respect to hellenisitc forces). The EB team's stance has long been that when roman authors make the claim that some troops are "armed in the roman manner" we should treat it very caustiously and that it most likely refers to troops fighting as thorakitai or in some other celtic influence manner (i.e wearing mail).

EDIT: It is important to note that it doesn't often if even mean fighting in a maniple or using a gladius.