Can I use 2.5v bulbs in a holiday light string designed for 5v bulbs?
My guess is yes - they'll just draw whatever current makes sense, and since they're lower voltage won't generate more heat.

No, that's not how it works.
Bulbs that are made for 2.5V can't be supplied with 5V, they'll draw twice as much power and burn very bright, but will probably burn out very quickly.
If you underpowered the bulbs - used a 2.5V power supply on 5V lights - that would likely just make them glow dim - but they'd probably last a long time.