Will this ramp stay in place on a concrete pad or do I need to secure it?

The ramp is designed as a portable unit. On level concrete it can sit freely, but securing it with weights or anchors is recommended if it'll be in a high-traffic area or outdoors where weather or foot traffic could shift it.

What's the weight capacity?

Weight capacity is not specified in the product data. Contact support with the user weight and mobility device specs to confirm suitability before purchase.

Can I cut or modify this ramp to fit a different rise height?

Aluminum can be modified with proper tools, but cutting or welding will affect the structural integrity and void any support. The 2-inch height is fixed as designed — if you need a different rise, a different ramp size may be a better fit.