Let's assume for the moment that we're not alone and there is indeed more life out there. How alien could it be?
Is it likely that aliens will be humanoid with funny faces (like Star Trek) or is it feasible that the could be creatures out there made out of light, sound or super intelligent shades of the colour blue?
How different to human could intelligent life be?
I suggest Stephen Baxter's short story collection Vacuum Diagrams where he describes (in a fictional way) many truly alien forms of (sentient) life. Among the most exotic is a (mostly) sessile alien form consisting of superconductive currents in the cold of Pluto; life living as an interaction between first order axiomatic systems of mathematics; the Qax being a system of interacting vortices (first in gaseous swamps and later in vacuum quantum fluctuations); photino life forms composed of dark, non-baryonic matter. Human descendants living on the surface of a (shielded) star in >20G seem almost normal in comparison.
When you talk about non-physical creatures (made of sound or shades of a color) you quit hard science behind and step in the realm of theology/magic. In essence you are implying that such creatures would basically be a soul, with extremely little presence in physical universe.
Life as we know it on Earth has one fundamental property: releasing energy stored in chemical compounds. If you look at green plants (which produce energy storing compounds) these too, have a respiration system (releasing energy). All single and multi-celled life, all aerobic and anaerobic life, all life in seas, mountains, air, plains, jungles, deserts ... all have some form of respiration.
When we take this "principle" to alien life, we are stuck with physical creatures like ourselves. If souls could exist free of bodies, we might have had "beings" made of sound or light here on Earth too since they would not have competed with physical beings in the famous war of "survival of the fittest".
But who knows there are such beings here on Earth? Dwelling with us, yet we being completely ignorant of their presence. Maybe soul is a truth and souls could exist without a body. Who knows?
Once again, I must say that such creatures could exist, but their existence would not come under the realms of science as it stands under present form.
Now to the other part of your question. When we talk about human-like life forms (Tharks and all), we are expressing more of our wish to find life forms like that, instead of expecting to find such life forms. Look at Earth. How many creatures resemble humans here? Some orangutans, gorillas and chimps maybe. Even these are far inferior to us when it comes to intellect and depth of communication.
While human-like forms could exist (there is no law prohibiting their existence), but the chance of finding such forms is extremely low, once you do the math about chemical composition of our bodies and the long, winding evolution journey we have walked, and survived.
Invertebrate insects like grasshoppers evolved bodies separarely from vertebrates, yet have bilateral symmetry with a "head" containing a mouth, and limbs farther down on the body.
It's a natural design that will be repeated.
Contrast with cephelapods, which are not put together how people think if you consider the cartoon representations. So that's not a universal solution even on Earth.
An oak tree doesn't have a head at all. So live is diverse in form. A macroscopic creature will have parts that interact with its environment in a manner of supporting its existence.
Large life forms will be made of smaller units, like cells. Exotic life not based on chemical processes still need "cell" units that fullfill the requirements of life. Magnetic vortices for example would need a myriad of distinct types with complex interactions; I.e. be just like chemestry. If you want to handwave and leave out details it must at least clearly be complicated in this way.
