Are there certain Pokemon only available on FireRed as opposed to Leaf Green like in the original two games?

yes

Exclusive to FireRed:

Arbok

Delibird

Ekans

Electabuzz

Gloom

Golduck

Growlithe

Oddish

Psyduck

Qwilfish

Scyther

Seadra

Shellder

Skarmory

Weezing

Wooper

Exclusive to LeafGreen:

Bellsprout

Kingler

Magmar

Mantine

Marill

Misdreavus

Muk

Pinsir

Remoraid

Sandshrew

Sandslash

Slowbro

Slowpoke

Sneasel

Staryu

Vulpix

Weepinbell