Do I need hose clamps to secure this?

Yes. Plan on two clamps — one on the 2.50" end and one on the 3.00" end. Step clamps (worm-drive or T-bolt) work best for smooth seating on tapered transitions.

Can I use this for both hot coolant and cold intake air?

Silicone handles both temperature extremes, but confirm your specific application pressure rating first. If you're running boost or high-flow intercooler lines, the 3-ply rating should work — just make sure clamp pressure is even across the transition.

Will this hose fit my existing hard pipe or metal elbows?

The IDs and ODs are sized to standard automotive hose specs, so it should slip over or into common fittings. Dry-fit everything before clamping down to confirm a snug, reliable seal.