Yes. The renders and animations you make with Crawlers are your own work, commercial projects included. The add-on code, the demo scenes and the assets are released under GPL-3.0-or-later.

It depends on the creature and your computer. On the development machine the simulation took about 64 ms per frame for 150 earthworms, 142 ms for 150 cockroaches, 286 ms for 300 ants and 41 ms for 20 snakes. For live interaction keep the count modest, lower Hair Density and Substeps, and bake before scrubbing; raise the count for final renders.

Yes. Raise Worm Length and Thickness on the swarm. Every behaviour is measured in body lengths, so a 90 cm giant worm moves like a real worm blown up. The included Undercroft demo scene does exactly this with glowing giant worms.

Yes. The creatures are real geometry with procedural materials, so they render in Cycles and EEVEE. The viewport shows lighter geometry and a third of the hairs while you work; renders always get the full detail.

The swarm is a simulation, and Blender does not advance an unbaked simulation zone during an animation render. Click Bake in the Crawlers tab first (it runs in the background with a progress bar), then render. The viewport plays the simulation without a bake.