Due to the nature of this campaign, each player is allowed to have two active characters at any given time. Each character must fulfill the rules as laid out on the Character Generation page. Each player may only play one character a session, meaning that while you may have a choice before embarking on each adventure, for the entirety of that adventure you may only play one character.
Playing a race with > 10RP:
When creating a character with a high starting race point (RP) value, the character will suffer a limited XP penalty to offset the racial advantages. To determine the XP penalty of a race (if any), simply subtract 10 from the race's RP. The difference, if any, is the % XP penalty the character suffers on all incoming XP. Each time the character levels, this penalty is reduced by 5%. When it is reduced to zero or lower, the penalty is gone forever and the cost of the race's advantages have been paid.
Natural 20s on skills:
Rolls of Natural 20s for skill checks are not automatic successes. Natural 20s always provide a critical over-roll attempt to determine exceptional results. If the combined result of rolls cannot reach the target DC for the action, the action still fails.
To determine the true value of a Nat. 20 roll, roll again. Subtract 10 from this second roll, then divide by two. Add the result to your initial Nat. 20 skill check. This is your final result. If your critical over-roll was also a Nat. 20, instead add the full value (20 plus your skill modifier) to your initial skill check (Which should also be 20 plus your skill modifier) then start again at the beginning of this paragraph.
Here, I'm somewhat stealing and adjusting a concept from other editions of DND. The basic concept is that you as players may be rewarded with something called 'Inspiration Dice'. These are dice of some magnitude, IE d4, d6, d8, etc. that players may apply to any roll they want for an additional bonus. These dice will most often be rewards for particularly well performed social encounters, for an especially innovative idea, or an interaction in which a PC performed remarkably well to character (Protip, it helps if your character has a defined identity for this, rather than being lumpy oatmeal). See here for a way to reliably gain inspiration dice. The score on the dice you get is entirely dependent upon what the DM thinks is appropriate at the time.
For example, if Liam is making an important stealth check, but only rolls a 3, and with his +7 Modifier, only ends up with a 10 total on his stealth, he may choose to utilize one of his inspiration dice. Liam got a d10 Inspiration dice last time he played for an especially well role-played encounter with an NPC, he decides to utilize this for his roll. He lets the DM know he's going to use this inspiration, and rolls the d10 and gets a 8 on the roll, great! He adds this to his stealth for a total of 18 on his stealth check, much better than his previous measly 10.
To further the role of luck a little bit in the determining of this game (as though the dice don't do that enough already), every character created has a 5% chance of possessing some valuable heirloom in addition to their starting gear. The exact nature of this heirloom is up to the GM, but they are typically magical in nature and innately useful. Some grow with the skill of the owner, some unlock new abilities with either time or strange arcane rituals, some don't change at all and merely begin at their full potential. Regardless, heirlooms tend to be intrinsically tied to their owner's backstory and history, and provide some sort of special boon to their wielder. The GM will roll for heirloom chance and the effects and properties of the heirloom, and then inform the player of what they've received.
Should you wish to play an evil character, that option is available, but only hesitantly. DND is, at its heart, a group venture. Evil PCs who disrupt group cohesion to an excessive extent will be dealt with in and out of the game at the discretion of the GM. This includes theft from other members of the party, needless or senseless back-stabbing, etc. While the occasional bit of such things can make for interesting roleplay experiences, constant or pointless engagement in creating party strife isn't fun for anyone. In addition, do not assume that since you are PCs and members of the group of adventurers that your character will be immune to repercussions should they decide to break the laws of the land.
Should anyone choose to play an Anti-Paladin, be warned that in my game, Anti-Paladins are made distinct from the default Paizo version through one key feature — Anti-Paladins are Lawful Evil, not Chaotic Evil. Any spells or abilities of the default class that specifies Chaos are altered to Law, and vice-versa. Anti-Paladins follow a strict code of Law, and accept the tenets of their deity to the letter, to the best of their ability. Straying from this path will cause the loss of Anti-Paladin abilities in the same way that a Paladin who broke their code might.