The floor plan is essential, and it must be to the same scale as the elevation. Both of which will be set-up on your worktop in a particular manner depending the type of perspective been drawn.

There are 4 basic types of perspectives. Those being 1, 2 & 3-point plus a multi point perspective. There are also different versions of the axonometric projections. However, these are not ‘true’ perspectives, as they do not have vanishing points. As a variation to these, there are transparent and exploded projections or perspectives that are so designed for specific purposes.

The only point where the perspective is to a true scale is on a line referred to as the picture plane (PP). Where the picture plane is position is a matter of choice. For example, if the PP were placed in front of the object it would increase in size. To make the object look smaller the PP would be placed behind the object. An alternate way to create a 3D image to scale would be to draw it as an axonometric projection.

To draw several objects in the same perspective you will need to use the multi-point method. Each object will have its own set of vanishing points that must be setup in relation to a common picture plane. The same can be said of complex objects.

Yes any perspective can be drawn without an elevation. That is providing you have the dimensions to measure out the required heights.

The viewing distance is a matter of choice. The nearer you are to the object the bigger, and conversely, the further you are the smaller the object. However, in a 2-point perspective the distance from the object should be approximately 2 times the width of the visible object. This is not a rule of thumb but merely a recommendation.

A fundamental rule in all perspectives is that the horizon is always at eye level. In 2-point perspectives this is usually within the height of the object. Whereas in 3-point perspectives the horizon can be above the object, known as a bird’s eye view. With a worm’s eye view the horizon is below the object.
Generally the horizon is on the level. But in some cases it can be tilted. For example, a view of an object as seen from a banking aircraft.

Perspective in art is important, but not essential. It could be put down as being artistic license or endemic to a particular style. Consequently, the artist can do what they see fit. Quite often in surrealism the perspective is deliberately manipulated to create extraordinary effects or impressions. However, if you wish to work in the realm of realism a good sense of perspective is foremost.

In 2-point perspectives this is usually within the height of the object. Whereas in 3-point perspectives the horizon can be above the object, known as a bird’s eye view. With a worm’s eye view the horizon is below the object. In the case of tall objects such as a view of Manhattan the 3-point method could be used. The parameter to switch to a 3-point is determined by the line of vision. Are you looking up at the object, or are you looking down?

This is drawn as a multi-point perspective where the cylinder is divided into equal segments and the different vanishing points are setup for each segment.