Language is both rigid and flexible. The number of permutations of the 26 letters in the English alphabet is huge. Even if you stick to the rules and avoid creating words like 'kfbjskjdfh,' that still leaves billions of words like 'hurfadulous,' 'timbarosity,' and 'smit' waiting to be called into action.
That's what makes it flexible. What makes it rigid is that the number of new words created every year is relatively small. For all that some resent the inclusion of 'blog', and 'leet' in the modern dictionary, these represent just the merest sliver of a fraction of what could be dreamed up. Despite appearances, the dictionary's entry requirements are extremely strict.