Is it in fact true that the faster the song is that the less reverb time should be applied/the slower the song the longer the reverb time should be used?.....Also is the same true for the amount of instruments used?..Like if a song has less instruments used in it the more reverb should be applied, and the fuller the track is the less reverb should be used?...I hope I put these questions in a way that's easy to understand...Thank you.

Not really... the effects you apply should be like adding spices to food... they should enhance, not take over (although if you come up with some interesting NEW effect that becomes a feature or hook of the song, it can "take over!")

But in general terms, the effect you choose should serve the song appropriately..... in specific terms of reverb, it's probably true that a slow ballad can have longer reverb times, but in actuality, for ANY time-based effect, delay/room/pre-delay/chorus times should be set as a function of the song's tempo.
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Bruce Valeriani - Mix Engineer

Reverb should be in time with the song played, it should be dead on with the beet, and cut off at mid point to the next. This way it gives the impression that the sound is larger than it is. You can also send it separate to each speaker so only one beet is affected on one side at a time, this will spread the sound out and keep it punchy down the middle, but as always, if you notice it having a mass effect, back it off.

I really like the food analogy used above, I think that reverb is a "spice." Remember that (this goes with any effect) you're trying to compliment the sound of the song. Play to what the song FEELS like. I have a few bands that come in that play speed metal, but they use really SLOOOOWWW reverb and delays, and it works like magic for them! So, there isnt a SET fashion for reverb or delay, just use it as little as possible, but as subtle as possible. I dont buy CDs for reverb, I buy them for the band.

It really entirely depends. If you don't want the effect to be noticeable, turn it up to where you can hear it, and then down just a bit. But a lot of people will use reverb as more than just an effect, which can be fun. You can even change the setting in the mix, which is great for swells and whatnot. A lot of instrumental guitarists (I'm mainly thinking of Joe Satriani though) use a lot of reverb and delay, relatively low in the mix, but very long. I play with it a lot.