No, khoms was obligatory at the time of the noble Prophet (s). The root of khoms is mentioned in the Qurâan as says:
“Know that whatever thing you may come by, a fifth of it is for Allah and the Apostle, for the relatives and the orphans, for the needy and the traveler, if you have faith in Allah and what We sent down to Our servant on the Day of Separation, the day when the two hosts met; and Allah has power over all things.” (8:41)
Therefore, as I said, the khoms was issued in the Qurâan, but it was limited at the time of the noble Prophet (s) about everything that Muslims came over during their wars with pagans. Later, at the time of Imam Baqir (a), the verse of khoms was explained further and khoms was applied to some other things as well. You should know that at the time of Imam Mahdi (a), some verses of the Qurâan may be reinterpreted due to the new demands of Islam and Muslims.
Only the Prophet (s) and the Infallible Imams (a) have the authority to interpret the Qurâan and their interpretations must be accepted and their orders must be obeyed.
The system of Islamic jurisprudence is complex, yet accurate and needs detailed discussions. But we can say that khoms is an Islamic rule, according to the Shiâa, and it is not a new issue in Islam. It is rooted in the very Qurâan and there is no difference whether the Prophet (s) issued a rule or other Infallible Imams (a). According to some Islamic traditions, all the Infallible Imams (a) are from one light; so all of them share a single source of knowledge.