It has nothing to do with the plural. The main word is Fleisch and the other is Schwein or Rind. Together it's Schwein-e-fleisch (the 'e' is an epenthesis). In Rindfleisch it works without an epenthesis. There are also other epenthesises: Hund-e-leine (dog leash), Ansicht-s-karte (picture postcard), Freund-es-kreis (circle of friends). These epenthesises (in german Fugenlaute) are not always regular, but for example for main words with the endings -keit, -heit, -ion, -ung the epenthesis is mostly 's' : Krankheit-s-anzeichen (symptom of disease) If you add to the main word a verb, the epenthesis is always 'e': Les-e-buch (reading book) In substantives it's mostly (e)s or (e)n, rarely er or e. These epenthesises were already developed in Old High German.

Richtig. Der Plural von "Rind" ist "Rinder". Aber bei Lebensmitteln sagt man immer nur "Rind". Ich weiß nicht, warum es so ist. Man sagt "Schweinehackfleisch" und "Rinderhackfleisch". In Süddeutschland sagt man auch "Schweinshaxe".