...
1. What is the main difference between HT speakers and stereo speakers, if there's any?
2. Is it OK to use different sets of vintage speakers (exp: KEF, Spendor/Rogers, KLH, and etc.) with different impedance to fill the HT channels? Currently, I have about 5 sets of speakers not being used.
...
1. Theoretically, there should be no difference at all. Speakers are supposed to accurately reproduce the signal sent to them. In practice, since home theater systems are designed for using subwoofers, there tends to be less need for the main speakers to reproduce deep bass. After all, if you have the subwoofer anyway, why bother with having the main speakers do that, too? Of course, many people use small bookshelf speakers for stereo, so this isn't always going to be different in practice, either. Another difference is with speakers designed for different positions in people's systems. In the case of surround speakers,
some (though certainly not all) have been made dipole or bipole in order to produce a more diffused sound. In the case of center channel speakers, the ones designated as such are made to fit people's cabinets and not block the TV; the horizontal orientation is not ideal for sound (if it were, they would make the right and left speakers horizontal for audiophiles, instead of, generally, vertical).
2. Being vintage or new is not important. What may be important is how well or poorly they match each other. How okay that is depends on how much you care that the sound will be altered by the different speakers sounding different. When, for example, a sound pans across the front, the tonal quality of the sound should not be changed by different speakers; the difference should be one of location, not due to a different type of speaker with different characteristics reproducing it. In real cinemas, they typically have identical speakers across the front, so that the tonal quality does not change. In my case, I use identical bookshelf speakers for all positions (except for subwoofers, obviously), so that when a sound pans from one channel to another, I have perfect matching, with the difference being only that which is caused by the sound being reproduced in a different location. I used to have "voice matched" speakers, and I will NEVER go back to just that. My center channel speaker is oriented vertically, just like the right and left, at almost exactly the same height. I picked my TV furniture with this in mind, after deciding on the speakers I would use. So sounds panning across the front sound great. I use bookshelf speakers for two reasons. First, a very tall speaker in the center would block the TV. Second, I have subwoofers for the deep bass anyway, so I don't need any of the other speakers to reproduce deep bass. I filter the bass from all other channels and send it to the subwoofers.
Now, since you already have a bunch of speakers, I suggest trying them. You may be happy with the results. In which case, you can save a lot of money. Not everyone is as picky about speakers matching as I am, and so you may not need to worry about this. It really depends on
you whether speakers matching matters or not. You are the one who will be listening to it, not us. You are more likely to be dissatisfied with front speakers not matching each other than the rear speakers not matching, both because humans hear better in front of them than behind them, and because surround channels
tend to be for ambience and effects rather than the main sounds that you are hearing, but, again, what matters is how you feel about it, not what matters to others.
As far as the impedance goes, that does not matter at all if they match each other or not; all that matters is that the amplifiers are okay with all of the impedances. And differences in sensitivity should be taken care of by the channel balancing that needs to be done anyway, though it could be a problem if the differences were very extreme, beyond the capability of your receiver/processor/preamp to adjust.