Connecting 0.25 watt 8 ohm speaker to 3.5mm mini jack output from laptop/portable player
Safely: yes, you can expect all outputs on consumer electronics to be shortcircuit-safe, so lower impedances (down to 0Ω) are no safety problem.
You won't get much (or good) sound out of something that can only drive, say 32Ω headphones when you connect 8Ω speakers. It's simple as that: if the impedance of the speakers is lower than what the amplifier is designed to drive, the amplifier can't drive it sufficiently.