Can we use a high powered amp to a low power speaker (e.g. 250 watts poweramp to a 100 watts speaker)?
Low power speaker? if youre reffering to powered speakers, then no - an amplifier is not needed in that case.
If youre reffering to power handling of a speaker, then the answer is yes. The "100 watt" is what the speaker can hold for X time before the coils burn or the speaker is destroyed.
Amp & Pre Amp/integ amp matching with speakers are much more complicated than just checking the "watt" specs. You have to take into account the impedance loads, room size if power is needed (for low powered amps + low sensitivity speakers) and of course, prefered sound character (Warm/neutral/Bright - MOST IMPT IMHO).
Start with what sound you prefer - then limit your choices from there. Then try to look into the capabilities of the amp if it can supply the required "juice" for your prefered speaker.
Hope this helps.
Cheers