What strength light? What cfm is that fan? Are you going to use a carbon filter?
I run one of those 6" inline booster fans (260cfm) at 100% and it cools my 600w in a cooltube. I don't filter though and would need to upgrade if I did. The same inline fan easily cools my 250w on the lowest speed my speed controller goes to (30% I believe). About 40-50% and the tube doesn't heat up at all. 600w the tube is quite warm. Touchable, but warm
I will upgrade it next grow so I can run something on 50-60% speed and reduce a lot of the noise. The fan I have now sounds like a jet engine running at 100%
They make all kinds or reducers. I get mine at Lowe's.they all work fine. Airflow is airflow. However, in my experience, that particular fan is more for assisting a blower. regardless, with air conditioning or other improvised stuff you can do just fine. Notice the shiny tape around the fittings. It's the shizbot! It's reusable and completely heat resistant. I still use the O clamps because I paid for em' before I knew about this tape. When you are working by your self it makes things go easier.