Regards the Portuguese flank companies.

The regulations state both are grenadier companies.
They also include a page on the duties of the flank companies as skirmishers.
It is also the case that at higher strength, which most were in the field, the unit operated as two separate battalions each of 10 platoons, each with one flank company.

Perhaps in the early days at lower strength there was an informal name use of one of the flank companies as the light company - but not an official one, and as soon as the units were at higher strength would not have matched likely operational use either I think.

Perhaps this is hinted at in one of Beresfords 'orders of the day' which reminded the army that the flank companies were called grenadiers.

Jeff Lewis