is pre ico compulsory only for angel investors and not to the public
or there are no hard and fast rules for this?
Before pre-sale, some ICO also offers a private sale that is held privately prior to launching their ICO publically to gather investors they can be their own team/company members, friends, business partners or others. you cannot contribute in private sale.
Regarding your question, in pre-sale anyone can take part you can also take part because both pre-sale, main sale some also offers post-sale all are announced to the public. In pre-sale ICO usually, not all, the minimum investment can be very high but it will give you a high bonus if invested in pre-sale. In the main sale, the bonus will be less or no bonus.
Private or Exclusive sale do happen but just on rare cases.The best time to invest is on pre-sale state where discounts as its best and profiting would really be more worth off when the token or project succeeded.This is why its really very important on researching up on which project we should put money on.Some people do prefer on after launching on exchange where usually price do dumps when investors and bounty hunters are already profit taking and buyback later on which is a good spot to get or accumulate cheap coins.