Bulbapedia says the following regarding EV-enhancing items, for example this is the Power Weight:

>Promotes HP gain on level up, but halves Speed stat in battle. Adds 4 HP EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.

Is it true that you can fight ANY Pokemon and obtain those EV's for that specific item? Have I been wasting my time, can I just fight anything?

And if so, say I fight a Tentacool with the Attack-enhancing item, will I obtain ONLY Attack-EV's, ONLY Spef. Def-Ev's, or BOTH?

Sorry for the convoluted question, I just need an answer.