Reload-Cancelling refers to interrupting the reload animation, allowing you to be able to fire more quickly than letting the animation finish. Certain weapons, such as the dual-pistols of the Gunslinger, allow you to cut their reload animation almost in half by doing it.

The steps are identical for every weapon in the game:

1. Reload the gun
   
2. Wait for the ammo counter to update
   
3. Interrupt the reload animation with quick melee
   
4. Interrupt the melee attack by shooting

Since you are likely going to use quick melee a lot in this context, it helps to rebind it to an easily-accessible key, like an additional mouse button if you have one.

Since the ammo update always occurs at the same time for each weapons reload animation, it is beneficial to get a feeling for when this occurs. For example, the Kriss Vector updates when the magazine is inserted into the gun. If you get a feeling for how long the "necessary" part of the reload is, you can focus of what is ahead of you and not on your ammo counter.

Yes and no.
Some weapons, like the SG500, the Incendiary Shotgun or the M4 Combat Shotgun, reload by putting individual shells into the magazine tube. This means instead of one reload animation that updates the ammo count to full, they have many small animations, each of them incrementing the counter by one.

While you technically can reload-cancel on the last shell, the overall saved time is minimal in comparison to the length of the reloading process.

If you wish to speed up reloading those weapons, I recommend not to fire the last shell in your magazine. This skips one part of the animation where the bolt is manually operated and instead goes directly to filling the magazine tube.

Yes.

Yes, somewhat.
Some perks simply don't have a Tactical Reload skill, but those can still benefit from Reload-Cancelling. Other perks, such as Commando, have skills that are just much more attractive than Tactical Reload - in that case High Capacity Magazines.

Two notable cases are Support and Demolitionist:

• Support opposes High-Capacity Magazines with Tactical Reloading. From my experience, Tactical Reloading only truely benefits tube-fed shotguns as stated above. If you gravitate more towards the nagazine-fed shotguns, I recommend getting bigger magazines.
  
• Demolitionist opposes more damage with faster reloading and more stumble power. More damage is helpful against Fleshpounds and not much else. It might reduce the number of rockets required to kill one Fleshpound, depending on the number of players on the server. Faster reloads primarily affect the unskippable part of the animation, so if you are particularly troubled by not being to shoot quickly, I would recommend picking faster reloading.

I honestly don't know. I have been playing KF2 since the closed beta and I recall that there was a time where reload-cancelling did not work anymore - that is, quick melee was not able to be used before the full reload animation has finished playing.

Then however, during the Christmas update I think (this is a wild guess, don't quote me on this), they re-introduced reload-cancelling again.

I don't know if Tripwire is unaware of the technique, if they intentionally keep it around as something to discover or if they are going to remove it at a later point.

Good on you. This guide is aimed at beginners and intermediate players who don't have several hundreds of hours in the game.