I have used CR calculators to get an idea of the difficulty of this encounter, but I thought the collective knowledge here may provide me with a better answer than I could find myself, as the encounter is somewhat unorthodox and I have no DM experience.
I calculated the tyrannosaurus at CR6 and Akena at CR1, but have difficulty with it because CR doesn't seem to look at anything other than HP, AC, Average DPR, and Attack Bonus. Also, I'm unsure as to how much an encounter should be scaled up when CR assumes 4 PCs and I have 5-6.
The first big milestone in the campaign involves a creature somewhat of my own design, based off a tyrannosaurus and a sahuagin priestess, the dinosaur being essentially a mount for the priestess. The creatures are as follows:
AC 15 (with mage armor) Perception +6,
Passive perception 16
Blood Frenzy: Advantage on attack rolls against creatures with less than their max hp.
Akena has the Mounted Combatant feat.
STR 8 (-1)
DEX 14 (+2)
CON 10 (0)
INT 14 (+2)
WIS 10 (0)
CHA 16 (+3)
Spellcasting Ability - Akena is a 6th level spellcaster with spell save DC14, +4 to hit with spell attacks. She has the following spells prepared:
Cantrips - Poison Spray, Ray of Frost, Shocking Grasp
1st level (4 slots) - Mage Armour, Magic Missile, Witch Bolt
2nd level (3 slots) - Snilloc's Snowball Storm, Enhance Ability
3rd level (3 slots) - Tidal Wave, Wall of Water
Tyrannosaurus (MM page 80) statblock:
Passive perception 14
STR 25 (+7)
DEX 10 (0)
CON 19 (+4)
INT 2 (-4)
WIS 12 (+1)
CHA 9 (-1)
Multiattack: Makes two attacks: One with its bite and one with its tail.
Bite: (+8 to hit) 3d12+7 (reduced from 4d12+7) damage on hit. Target is grappled (DC 17 save.) Until this grapple ends, Tyrannosaurus can't bite another target.
Tail: Melee weapon - (+8 to hit) reach 10ft, 2d8+7(reduced from 3d8+7) bludgeoning damage.
The party consists of a Lizardfolk Barbarian, a Tortle Ranger, a Wood Elf Rogue, a Sea Elf Sorceror, and a Triton Druid (all level 5). They will be accompanied by a ranger NPC I've created that is slightly less powerful than them. The Barbarian and Ranger PCs are experienced players.
Is this encounter too difficult/easy?
This is meant to be the moment in the first third of the campaign where the main plot hook is partially revealed to the players, and I want the encounter to be difficult and somewhat nerve-wracking, ideally knocking 1-2 PCs unconscious but I certainly don't want a TPK.
The main thing giving me trouble considering the difficulty of the encounter is Akena, being mounted atop the tyrannosaurus and being able to spellsling in relative safety. I forgot to add the Mounted Combatant feat to her when originally posting this question. I've nerfed the damage of the Tyrannosaurus a bit to compensate. Should I have left the damage as is in the Monster Manual?
If I take the suggestion in the answer below and add 2 CR2 Allosaurus, That would take the CR of the encounter up to CR11. Is that something I should regularly expect my PCs to be able to defeat?