Yes, we commonly express the modifier/adjective of a plural noun that way because it is the grammatically correct way. It is also used with nouns with a “ma-”, “man-” or “mang-” prefix.
Maba
ít SIYA = HE/SHE is good/kind
Mababa
ít SILA = THEY are good/kind. (Note that “siya” becomes “sila”.)
In casual conversations or informal writing, some of us may incorrectly use the adjective in its singular form. So, saying “mabait SILA” is also common.
If the group is small, the plurality would normally be understood as true for everyone in the group. However, if the group is big, the attribute may be understood to apply only in a general sense or to refer to a significant majority in the group.
The word “mga”, which marks plurality, may also appear with it. That “mga”, though, is modifying the noun and not the adjective.
The following sentences all mean, “All their children are already grown-ups”.
MALALAK
Í* na ang MGA an
ák nila.
Ang MGA anak nila ay MALALAKI na.
MGA MALALAKI na ang anak nila. (Some of us might even unnecessarily add another “mga” right before “anak”.)
MALALAKI na ang anak nila. (“Malalaki” indicates the plurality, but grammarians may consider the sentence incorrect because of the absence of “mga” either before “malalaki” or “anak”).
A Filipino might say, “MATATANGKAD ang MGA Amerikano (Americans are tall)”, and it will be understood only to mean that in general, or based on the average, they are taller.
* “Malalaki”, on its own, will have the stress on its last syllable “ki”. But when “na (already)” follows it, we would pronounce it as if “malalaki na” is a single word and place the stress on the “na” instead.
The word formation is also used with nouns, but in all cases, the word is applicable to both singular and plural usage. In the following examples, the number of the subject or actors is the basis for knowing if the noun is used in the singular or plural sense. The “mga” may also be added when the noun is used in the plural.
M
ánan
áh
î siya. = He/She is a sewer/tailor/dressmaker.
(Mga) M
ánan
áh
î sila. = They are sewers/tailors/dressmakers.
(Mga) M
ánanangg
ól sina Jack at Jill. = Jack and Jill are lawyers.
Mandar
áy
â ang tindero. = The vendor is a cheater. (The root word is “daya”. We don’t repeat the “da”, but use "ra" for the second “da” instead.)
(Mga) M
ángingisd
â kami. = We are fishermen.