Normally, you should be 3rd level after first module, 5th after the second, 7th after the third, as indicated in the beginning of each module.
I've run this with 6 player party. First module runs adequately without too much modifications, I may have added a guard in one or two encounter but that's it. At low levels the firsts boss are hard enough.
I made a one-shot in Dalesko at the end of first module. Which gave my player a head start for the second module, which i ran as is.
I had to add some people in the third module with 6 players. There are some possible solo boss fight that can be too easy with more than four player, so you have to adjust the number of goons accompanying them.
I modified the ending with contents from the Starship operation manual, For instance, I could not resist adding an automated party in the last starship battle.
Have a nice campaign!
