Wa 'alaikum salaam.
I don't think Prophet Muhammad(pbuh) has given different laws for Shiya and Sunni.There is no such laws in Islam. Allah has given us one True Religion, and there is no different laws for different people
The answer is no. No imaam is required for a marriage.
"One day Prophet Muhammad(pbuh) was walking in the street. And a Sahaabah came in that way. Prophet Muhammad(pbuh) asked him about his perfume he had on him. The Sahaabah replied that he married a girl yesterday. Then Prophet Muhammad(pbuh) asked about Mahr...etc"
By the above Hadith, one Sahaabah married a girl without Prophet Muhammad(pbuh) knowledge and Prophet Muhammad(pbuh) didn't get angry for doing a marriage without him. This means There is no need for an Imaam in a Marriage