I vaguely recall hearing an (unwritten) rule in mallu says that one should not combine Sanskrit and Tamil roots in the same words. So one would say കല്പലക as opposed to കല്ഫലകം; പൂമെത്ത as opposed to പുഷ്പമെത്ത; ജന്മദിനം as opposed to പിറവിദിനം.
But we do have common words that seem to violate this rule:
കണ്മണി, കണ്മഷി, കാര്മേഘം Each of these words is formed from a Tamil and Sanskrit root; and I cannot think of a common alternative that is either pure Tamil or pure Sanskrit.
Which makes me think that the "rule" that I am recalling may not be correct. Or, maybe there are well-defined exceptions to the rule.
What do you think?