Growing again. Help with setup

Room size 1.6ft x 3.3ft x 5.8ft
600hps
6" inline fan
Carbon filter
White paint inside only

Aiming for 2 plants

Do I need one more inline fan?

Depends on ambient temps and the quality/CFM of the current fan. A 600 is a large amount of heat in a small space. Have you run the setup how it is for a point of reference? I would have more fans available personally that way if needed they are there. And remember to have a spare bulb on hand for the hps even if its just a cheapo.

if you have a dimmable ballast run it at 50-75 oor heat will be a bitch

If you get a cooltube, you can probably run your 600w @ 100%. As long as the 5.8 is the height. I run a 1k and my tent hits 75. I don't run my house A/C at all yet.