Can't a secondary attack be an "and" option in a full attack line? I would favor
Of course it can, I just wasn't sure I liked using that approach for the Cessirid, since the mouth-tentacles seem pretty short. The original description also only describes it using its tentacles to add poison to its bite attack. This grappling business is something we're basically making up out of whole cloth.
Attack: Bite +10 melee (10d10+6 and poison) or tentacles +5 melee touch (grapple plus poison).
Full Attack: Bite +10 melee (10d10+6 and poison) and tentacles +5 melee touch (grapple plus poison).
There's no real need to list the tentacles in the Attack Line, since a creature can use a secondary attack as a standard attack if it wants to according to the SRD.
So we'll be giving it a Trip with its bite attack and an Improved Grab with its tentacles then? e.g.:
Attack: Bite +10 melee (10d10+6 and poison)
Full Attack: Bite +10 melee (10d10+6 and poison) and tentacles +5 melee touch (improved grab plus poison)
Improved Grab (Ex): To use this ability, a cessirid must hit with its tentacles attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and affects its opponent with its poison.
Poison (Ex): A cessirid's mouth-tentacles constantly ooze poison. Any creature hit by a cessirid's bite or tentacle attacks must save against this venomous slime. An opponent must also save against a cessirid's poison every time the illithidae makes an opposed grapple check against them. The cessirid does not need to win the grapple check, since mere contact is enough to poison its foe.
Cessirid Slime: Contact or injury, Fortitude save (DC 18); initial damage sickened for 1d10 minutes; secondary damage 1d6 Dex and sickened for 1d2 hours.