1) Vector addiction always occurs tip to tail. Never change the direction of the vectors simply because you're adding them. However, once you have established the "resultant" you can break it down again into two perpendicular vectors, which you can then use the Pythagorean Theorem to solve for.

Another way you can do it is to break down each of the two original vectors into X and Y components, add the corresponding X and Y components together, and use Pythagorean Theorem to solve for the resultant.

2) Yes. Acceleration is a vector, so always indicate your positive and negative directions in your FBD. This is especially important when you're using g in kinematics too.

3) Frequency and period are the inverse of each other. That's why we write f = 1/T. Period is the amount of time for one rotation (measured in seconds usually), while frequency is the number of rotations per second (measured in Hz or s -1). Frequency can be both a decimal or greater than 1. For example, if the period of a wave was 2s, then the frequency would be equal to 1/T or 0.5 Hz. If the period of a wave was 0.5s, the frequency would be 1/T or 2 Hz.

Hope this helps!