[Speedslug]

My first thought is; Is there any possibility of getting the hood opening increased from 7 inches to 8 inches.
Lots of heating, ventilation and air conditioning companies can snip the hole size easily for a price.
They do it all the time and hardest part is getting the hood to them (much cheaper) or them to find a room in their schedule to come to the hood (read: house call $$$).


Any change in size effectively turns the entire rest of the ducting after the change to that size.
A smaller size will increase the speed of the flow through the restriction and puts strain on the motor doing the propulsion. Most motors can handle a little extra strain but it will shorten how long that motor will live.

If you then go back up in size to the size before the restriction the flow will slow down and some of the particles suspended in the flow will fall out and coat the inside of the ducting.

There is a lot of discussion about ventilation in the safety threads here.

Short answer; since you aren't setting up an industrial 24/7/365 factory then yeah, go for it.
If the fan is at the wall and outside is just an elbow the same size of the fan outlet you should be fine.
Keep the same size duct from the hood to the fan and then the same size from the fan to the outside dump.

If you get seriously addicted to molten glass you would be better off getting the hole on the hood increased and running the same size duct on the whole run.

Bigger CFMs is better when you can and don't believe any CFM marks on bathroom fans. They often flat out lie and aren't even a quarter as good as they say they are. They just move shower steam and farts so no one calls them on it.