Regarding Transmission line
Every wire is a transmission line when the frequency of the signal gets to the point where the physical size of the wire is comparable to the wavelength of the signal traveling down the line.

In normal circuit analysis, we make several assumption. First, we are using DC or low frequencies and we assume wires are perfect with no parasitics and no resistance to it. There are other assumptions such as no coupling between magnetic fields. I am talking about a first course in circuit analysis.

There is no hard and fast rule. Typically the rule of thumb is that if the wavelength of the signal is about 1/10th of the length of the wire, that you should consider it a transmission line. 

In practice this really manifests itself in 300MHz+, but in some cases 100MHz can also require some attention.