A round table is always best, whatever the numbers, if possible, and particularly with 6.

The placement is easy with six no matter what shape the table, just do: boy - girl - boy - girl and make sure no one is sitting next to their partner. As there are only two people of the opposite sex who you could sit next to you can't get it wrong. Also, traditionally the most important female guest sits on the host's right and and the most important male guest sits on the hostesses right.

I agree. A round table encourages people to interact with everyone on the table and not just the person sitting next to them. Also try seating them boy girl boy girl and DEFINITELY not next to their partner!