6.35 (or technically 6,35) is the same as .25 ACP.
It depends on the make and model of .25 caliber handgun, but, generally the magazine would hold at least 5 rounds with one in the chamber for a total of 6 rounds. The correct terms for "bullets" are "rounds" or "cartridges", a bullet is the projectile that exits the firearm.
NO. The .22 is a RIMFIRE cartridge, and is smaller in diameter than the .25 ACP- which is a center fire cartridge.
6.35mm is also known as the .25 ACP.
It is possible, however "not preferable" is drastically understating the seriousness of it. If you put the wrong caliber cartridge in a handgun, you can possibly damage the gun and/or hurt yourself, so it isn't "not preferable." INSTEAD, it is a really, really bad idea that you should never try.
25-65 USD
S&W never made a 25 ACP handgun for retail sale.
That question can't be answered as asked. It depends on the brand and model. Check with the manufacturer.
Small handgun
No. The .22 is smaller in diameter than the .25 and would be too small for the chamber of a .25. Generally speaking, you should never try to fire any cartridge in any handgun other than what it is designed to fire. While it may be possible to get a cartridge to fire from a different gun, it may cause damage to the gun or the shooter. Additionally, I forgot to mention that the .22 is a rimfire cartridge and the .25 is a center fire, the the firing mechanism is slightly different in the two guns.
25 ACP handgun
small .22 or .25 caliber handgun