(12 points)
Let $\{a_n\}$ be a sequence with $a_1 + a_2 = 2$.
(1) If $\{a_n\}$ is an arithmetic sequence and $a_1 + a_3 = 5$, find the general formula for $\{a_n\}$.
(2) If $\{a_n\}$ is a geometric sequence and $T_n$ denotes the sum of the first $n$ terms of another related sequence with $T_n = 21$, find $S_n$.