One difficulty to bear in mind, if you do want to use "ain't", is that non-standard formations tend to occur not singly, or in random combinations, but in restricted compatible clusters. They also tend to require a certain accent and intonation.
For instance, a modern "east London" use of "ain't" would tend to be supported by double negatives, the use of "was" instead of "were", a particular accent, etc., while the 18th century "aristocratic" use would have had an entirely different cluster of accompanying formulations.
If you use a form such as "ain't" in isolation, therefore, among standard speech components, and with a non-native accent, it won't sound natural or impressive: quite the contrary.
It's a little like cloth caps and gaiters, in that respect.
"whom" > "who" (as the pronoun here is the subject of "use", not the object of "know").You had better let me know whom of native English speakers use it.
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