Best practice for short sentences in a deep learning network
Yes, when you have different sequences with different length you can pad all the sequences so that they have all the length of the longest one. is a good practice and doesn t impact the learning mode. Another solution could be online learnig. With online learning you give to the model 1 sample at the time (batch_size =1) and in keras you can train the model with batch with different lengths. 

With online learning your computation is slower, but theoretically your model need less epochs to learn.

If you want to pad you should pad with all 0s and not random values.