Is it possible for creep to spread in the installation tileset? at the very least, for it to spread on substructure floor?

No, you can't build on installation, so creep won't spread anywhere.

There is a buildable tile in Installation, and creep will spread on it, but since creep tiles don't exist on the tileset, it will create null tiles instead of creep as it spreads. This has a neat effect of creating an area where you can have units simply walk onto null tiles, after which they can walk on anything (as per the usual behavior from moving them onto null tiles), though they'll more than likely get stuck on whatever building you're using to spread creep.

Any substructure tile from 0016.00 to 0048.00 on the tile palette is buildable.

also by the way I have a small issue with SCMDraft2. everytime I end up putting terrain on the edge with the brush it freezes for a bit.

Alt tab and tab back in. Works for me most of the time, otherwise just let it sit there. It's a known issue with no known fix.