It really depends on which style of scifi you are going for Tebrown. Cutting high-tech like  Star Trek/Star Wars, Space Gothic with Warhammer 40K, Industrial like Firefly, something more alien, or some kind of mix?

As important is sizing, one inch floor tiles verses 1.5 inches. I started out and stuck with one inch tiles.  On the downside they are more labor and technique intensive.  The plus is there are allot of creative options.  1.5 inch tiles have a strong advantage with production speed and less difficultly when building, lower engineering tolerances as it were.

My six preferences for genera specific would be (with the one inch tiles approach);

Cutting Edge High-Tech
Starship Wall Mold #301, Cargo Bay Accessory Mold #302, Med Lab Accessory Mold #303, Starship Deck Mold #270, Pipeline Floor Mold #273, Industrial Accessory Mold #326

Space Gothic
Pipeline Floor Mold #273, Cargo Bay Accessory Mold #302, Machinery Builder Mold #327 (mixing in higher tech decoration along the walls),  plus three wild-card mold picks.

Scifi Industrial
Pipeline Floor Mold #273, Machinery Builder Mold #327, Industrial Accessory Mold #326, Pipe Mold 3/8" - Mold #321,  Industrial Edge Mold #325 (I don’t have one but others shown just how practical and sexy it is), Machinery Builder Mold #327

When starting out I tried really hard to keep down the number of futuristic molds.  That lasted about one year.  Slowly adding in more as new ones came out, right now fighting off the urge to get the Industrial Edge mold.  My collection of the scfi side I’m using eleven now, maybe twelve by August.  From a craft/hobbyist side be ready for at least eight molds, don't be surprised when it has to be ten, and try not to choke as others tempt you.

On a few different paths...

If the goal is getting out some terrain for more practical reasons and less hobby/craft I’d consider Large Catwalk Mold #276, Large Grate Mold #277, Large Deck Mold #278, Large Grate Accessory Mold #279, Industrial Edge Mold #325, Machinery Builder Mold #327 (or Industrial Accessory Mold #326).  All for a significantly faster project, cheap, and less hassle to fit into a nice generic science fiction setting.  Or as we used to say, “wham bam thank you ma'am.”

Answering the two mold question.  For causal experimenting the Large Catwalk Mold #276 (floor and a door) and Industrial Edge Mold #325 (wall definition) are well documented in projects on the main site.