Is an infinite tape alphabet for TM valid?
Turing machines have a finite tape alphabet.

You can think of a generalization of Turing machines with infinite tape alphabet, but there are two problems:


The description of the machine is no longer finite.
Such a machine can decide any language over $\{0,1\}$ (exercise).


There are ways around it - for example, we might require the rules to be finitely specifiable in some specific form. The resulting model will then be equivalent to Turing machines (in terms of computability).