There is no such thing as a 形容動詞 which 'takes の' as opposed to な. The dōshi component of the name reflects the fact that na inflects like a verb, albeit in a very limited fashion---we only find -na, when used attributively, and -ni when used adverbially, e.g., 華やかな装い, 華やかに装っている. As has been pointed out in posts elswhere this na is the remains of the literary naru.
Normally, keiyōdōshi cannot be used as subjects or objects as they are not nouns, e.g, *benri ga/wo, and the normal (abstract) noun formation would be to add -sa, e.g, from benri na, 'convenient', benrisa 'convenience', but there are examples of keiyōdōshi that fuction as abstract nouns as well, e.g., teinei na taido, teinei ga daiichi ('Politeness is first', a slogan).
The examples of 普通, 大勢 are nouns. If they occur in a construction with の, such 普通の態度 or 大勢の人 they should be construed as the predicate of a sentence with the copula as verb, hence, ‘ōzei no hito’ is to be construed as 'ōzei de aru tokoro no hito' 'people such that they are a great number', 'futsū no taido' as 'futsū de aru tokoro no taido', 'an attitude such that it is normal'. (This construction, Verb + tokoro no, appears to have come from the translation of the English relative pronouns, but it is sometimes used in scholarly texts, or the like, to make an attributive relationship explicit, which is why I have used it here.)
Words like 特別 and 特殊 form another class in that they can take な when used attributively, like keiyōdōshi, e.g., 特殊な技能, but may also take の, 特殊の技能.