Hi
One of the things you need to investigate is the carrying capacity of the Doblo people carrier. When I worked for Citroen the Berlingo came in 600 or 800 kg payload whereas the Multispace was about 450kgs. The reason for the lower payload on the Multispace was for passenger comfort where this wasn't a consideration for the van. Extra seats are extra weight that comes off the payload.
You need to contact Fiat technical and ask the question to be sure.
Spruce