Calculate the ratio (AD)/(DF).

We are given that $\frac{AD}{AF} = \frac{3}{5}$.
This implies that $AD$ represents 3 parts of $AF$, and $AF$ represents 5 parts.
Therefore, $DF = AF - AD = 5 \text{ parts} - 3 \text{ parts} = 2 \text{ parts}$.
So, the ratio $\frac{AD}{DF} = \frac{3}{2}$.

We are given $AE = 12$ units and $EC = 8$ units.

Since $\frac{AD}{DF} = \frac{AE}{EC} = \frac{3}{2}$, by the converse of the Basic Proportionality Theorem (also known as Thales's Theorem), the line segment $DE$ is parallel to the line segment $FC$.
Therefore, $\boxed{DE || FC}$.

2. If $AB = 14$ units, calculate the length of $BF$.

The statement "E on AC // FE" is interpreted to mean that the line segment $FE$ is parallel to the side $BC$ of $\triangle ABC$. So, $FE || BC$.

Since $FE || BC$, we can write the proportion:

$AC$ is the sum of $AE$ and $EC$.

We have $AB = 14$ units, $AE = 12$ units, and $AC = 20$ units.

To solve for $AF$, multiply both sides by 14:

Simplify the fraction $\frac{12}{20}$ to $\frac{3}{5}$:

The length of $BF$ is the total length of $AB$ minus the length of $AF$.

The length of $BF$ is $\boxed{5.6 \text{ units}}$.