I wouldn't mix oil and acrylic under any circumstances. If you're going to paint in oils, then purchase canvases or panels prepared with an oil-based recipe, or (even better) prepare your own supports. If you work in acrylics, then it's ok to paint on commercial supports prepared with acrylic gesso.
To varnish a finished oil painting, use an oil-based picture varnish (artists differ in the amount of drying time needed before applying varnish. Variables include humidity, painting mediums used in mixing and thickness of the paint film. Some varnish after a few months, others wait an entire year.) To finish off an acrylic painting, apply one or more coats of clear acrylic medium.
Many amateur artists oil paint on top of supports primed with acrylic gesso. Many "mixed media" artists finish an acrylic under-painting with oil paints. If the painting has been totally over-painted with oils and glazes, then it's an oil painting in terms of the type of varnish that should be applied. You cannot apply acrylic paints on top of oil paints, nor can you coat an oil painting with acrylic medium; the oil paint underneath will not dry evenly, the acrylic will not adhere properly as the oil paint cures and solvent/oil molecules migrate to the surface. The acrylic film will crack, offering no protection for the painting, not to mention an unsightly appearance.
