1/ You generally may not initiate a grapple with a creature 2 size categories larger than you. The Improved Grapple feat allows you to attempt to grapple 2 sizes larger, and Greater Grapple lets you attempt 3 sizes larger.
2/ In a grapple, a creature with 10' greater reach than their opponent is not considered grappled. A creature with 5' reach is not considered grappled by a creature with 0' reach. (Note that the -20 penalty option still applies for other cases)
3/ When a grapple starts, a creature with reach draws the target 5' closer to it. The grappled target may attack that appendage, even if they cannot reach the main creature. Allies of the target may also attack the grappling appendage if they are adjacent to the target.
4/ A pinned character who manages to make a check to remove the pin becomes grappled, unless their foe releases them. If the character has iterative attacks and are successful in escaping the pin, they may immediately make another attempt to escape the grapple (and subsequent attempts based on iterative attacks).
5/ Unless otherwise stated, a creature with the Swallow Whole ability can only swallow creatures up to 2 sizes smaller than it. It may only hold one such creature at a time (unless otherwise stated), with smaller creatures counting as half the size of a the next size up (ie 1 large = 2 medium = 4 small = 8 tiny). They may not swallow more creatures until the current creature(s) is digested, escapes, or is destroyed somehow (ex in a Remorhaz or Furnace Golem).
6/ Feats which modify grapple attempts, or CMD vs grapple attempts, also apply to maneuver attempts to escape a grapple/pin
Channeling energy to Harm: Mindless creatures do not get to make a Will Save to half the damage vs channeling energy, unless they have Channel Resistance.
Summon ability of monsters: If a summon special ability specifically calls out a spell, it takes the same action type as the casting of the spell. If it does not (ex evil outsider summon abilities), it takes a standard action.