The suffix for turning a singular noun into a plural noun is கள். Since கள் does not begin with a vowel, none of the special previously learned rules for adding suffixes apply. In addition, words that end in certain letters change while becoming plural.
This may seem like a lot, but it isn't. Most nouns fall into the last category, which is simple. One syllable words that end in a vowel sound are actually one-letter words. Words like this are பூ, ஈ, தீ, but there aren't too many. There also aren't many short vowel sound, one-syllable words ending in ல் or ள், but some are பல் and கல்.
So although all of the rules may come in handy, it is typical to use the first rule and the last rule the most.