If AOP/BOI do not have taxable income, can they use ITR-8 instead of ITR-5? What about a firm? Can they also use ITR-8 instead of ITR-5?.

It is the requirement of filing the return of income which would decide the issue and not the amount of taxable income. ITR-8 is to be used if the taxpayer is not required by the law to file income tax return but has FBT liability.