Я играющ - sounds like you made a short present participle there but it is not a word, sorry.

Она красива - short adjective
Она красивая - normal adjective

Он ужасный - normal
Он ужасен - short

The short adjectives are used only if there is no noun to the adjective

Она красива - okay. Она красива женщина - wrong, only она красивая женщина.

Moreover, the correct usage of short adjectives is a very complicated topic because sometimes it's inapproriate to use a short adjective at all or an adjective just doesn't have the short form (like маленький).

Yes, if a noun looks like an adjective it changes like an adjective. Examples:

Следующий - Позови следующего (пациента)
Столовая - В столовой (комнате)

Hope it helps