Question. Is there an integer $n_0 \geq 2$ such that $$\left\{\frac{c}{rad(abc)^{n_0}}: a, b >0,\; c=a+b,\; \gcd(a, b)=1\right \}$$ is bounded?
The abc conjecture can directly deduce this conjecture.
Question. Is there an integer $n_0 \geq 2$ such that $$\left\{\frac{c}{rad(abc)^{n_0}}: a, b >0,\; c=a+b,\; \gcd(a, b)=1\right \}$$ is bounded?
The abc conjecture can directly deduce this conjecture.
You Question 1 is a weak form of the $abc$ conjecture that is known to imply asymptotic Fermat, among many other applications. So no, it is not known to be true, but it is a well-known conjecture, and thus unfortunately not an appropriate question for this site. For your second question, if there is a solution, note that $c=rad(abc)$ implies that $c$ is square-free. But then $rad(abc)\ge c$, with equality unless every prime dividing $ab$ also divides $c$. But if $p$ divides $a$, then $p$ divides $c$, which (from $c=a+b$) implies that $p$ divides $b$, contradicting your $\gcd(a,b)=1$ assumption. So it seems that your set in Question 2 is the empty set.