For quick reference, here's a simple chart, describing most of the major art media, that might help you determine which can be combined in mixed media artwork. Generally speaking, all water media can be combined in various ways. All oil-based media are also generally compatible. Oils can be applied on top of water-based washes, but you cannot apply acrylics on top of oils or any waxy surface. This chart is from Exploring Color Revised.

See also my paint FAQ.

Medium	Binder	Diluent/Solvent	Characteristics
acrylic	acrylic polymer emulsion	water/denatured alcohol (limited use)	fast drying (dries darker); opaque or transparent
alkyd	oil modified alkyd resin	oil medium/pure gum turpentine, mineral spirits	similar to oils, but fast drying; compatible with oils; opaque
casein	milk solids	water	fast drying; opaque; matte
colored pencil	wax, gum	mineral spirits, colorless marker	applied in layers; waxy; buff for shine
gouache	natural gum	water	fast drying; opaque, matte
gouache (acrylic)	acrylic	water	same as gouache; dries water resistant
ink (pigmented)	gum, shellac or acrylic emulsion	water/denatured alcohol	fast drying; transparent, brilliant color; use lightfast only
oil or oil sticks	natural oils (linseed, poppy, safflower)	oil medium/ pure gum turpentine, mineral spirits	slow drying; opaque
oils (water miscible)	modified linseed oil	pure gum turpentine, mineral spirits/ water for cleanup	slow drying; loses water miscibility if too much oil is used
pastel	weak gum solution	only for water soluble soft pastel	brilliant pure color; opaque; soft or hard
pastel (oil)	natural oils and wax	pure gum turpentine, mineral spirits	opaque pastel effect with no dust
tempera	egg yolk	water	fast drying; opaque; translucent layers
watercolor	natural gum	water	fast drying (dries lighter); transparent; matte
watercolor pencils or sticks	water soluble gum	water	mostly transparent; wettable for wash effects