To Gravel or Not to Gravel, that's the question

Mulching is another mystery to me. When I first started gardening, I put down loads of bark mulch because I heard that it helps to retain moisture, makes your beds look neat, leeches beneficial elements into the soil, and I've seen it on commercial flower beds. Then I read that citrus should not be mulched because the bark might rot.

A friend gave me LOTS (I mean a pickup truck bed-full) of red lava pea-sized gravel. Wouldn't it look nice at the base of roses, and keep the roots warm? What a mistake. My Eglantyne, described by David Austin as one of his most satisfyingly robust cultivars, nearly died. But the Gardenia Veitchii right next to it, started to bloom right away and kept blooming, for two years so far. Was it the gravel, or sheer chance? Should I use the gravel elsewhere, around other tropical plants such as gingers and passionflower? Will it kill the volunteers that spring up around the hedges? What other plants like to have warm roots?