How easy would it be to make new races for a campaign and introduce custom mechanics/rule changes?

I'm starting a campaign and i'm looking to add a couple new custom races and a game mechanic and i was wondering how i would go about that. Do i just add it in or is there rules to follow?

any changes to the rules or additions are completely up to you there's no fast or hard rules to follow.

that being said my recommendation is to pay attention the the pro's and cons of the species in the books and try to balance them out in a similar manner for any species you make.

for custom game mechanics i can only say don't be afraid to balance them on the fly. don't let your players say "but you said earlier that it worked this way" if it makes the game too easy or too hard just explain that and change the rule. you might have to play around with it a little to get it to balance with your groups play style.

OK. thanks. Also I've been wondering with advantages and disadvantages for character creation...is there a limit? or is there only those allotted spaces on the character sheet that many because there was no room for more?

You are not allowed to take more than three Disadvantages. Though I should warn you, there should be no such thing as a 'free' or 'easy' disadvantage

The GM should be sure that each disadvantage will come up during play. A level one disadvantage should pop up every so often, while a level two disadvantage should be a constant pain to the player.