There are a LOT of Christmas albums from the 1950s and 1960s that are mixtures of pop and classical artists, with some orchestras tossed into the mix both classical (NYPhil with Bernstein, or Ormandy and PhiladelphiaOrch) and pop (The Melachrino Strings, etc.). Many sold by companies as promotional albums for customers (Firestone, Goodyear [may family had one of these], W. T. Grant Department Store). The classical artists are not lightweights either. Here's a list from the Firestone series: many of the classical artists were affiliated with the Metropolitan Opera: Rise Stevens, Brian Sullivan, Roberts Peters, Franco Corelli, James McCracken, Dorothy Kirsten, Leontyne Price, Nicolai Gedda; or Broadway: Martha Wright, Gordon MacRae, Julie Andrews; or Pop music: Vic Damone, Jack Jones, Vicki Carr, John Gary. Very often, those classical sopranos were singing either the Gounod Ave Maria, or Schubert's Ave Maria. Leontyne Price's version is especially fabulous.
