I want to make diy guitar combo for guitar practice. I have two speakers (6 and 10 inches) and don't want spend money for new one.
Is any difference between special speaker for guitar combo and common one?
Guitar cabinets are designed to give a completely different response from HiFi speakers. In particular, they cut away / smear out a lot of the high frequencies and transients. This is really important for distortion sounds (without something like a guitar cabinet response, these sound just extremely harsh, but not really powerful – rather thin†), and also just feels “right” for clean sounds.
If you want to build a guitar cab out of HiFi speakers, and want it to sound anything like a guitar amp, make sure that no tweeters are audible. These are completely counter-productive (unless you're using the amp completely clean, for acoustic-guitar like transparency). If the speakers have tweeters built in, you can try to just disconnect them (the crossover might not like this, but perhaps it would sound ok).
For three- or even four-way speakers, it might make sense to disconnect the bass drivers as well. Really, midrange is the most important thing for a guitar cab!
Apart from that – well, the impedance should be at least the amp's minimum rating (if you're using a tube amp, also not much more), and the power rating shouldn't be completely off either. If it's a tube amp, better make sure the speaker can cope with more than the amp's maximum rating. For a transistor amp, this isn't so important, though you should better not crank the amp all the way up...
No difference, except that with ANY speaker usage, the wattage and the impedance always need to be considered and put into the equation. Otherwise, a speaker is a speaker, given these parameters.
Broadly speaking, Hi-fi speakers and amplifiers are designed to sound accurate. Guitar amps and speakers are designed to sound interesting.
But for a practice setup, I wouldn't worry about this too much. Just make sure you don't turn up loud enough to damage the speakers. Increase volume gradually. You'll hear when they start to suffer. Then turn back a notch. That is your ABSOLUTE limit, unless you want to buy new speakers.