Learn English – Verbs “may” and “let” used to wish somebody something

It seems as though you have already looked up these words, so I will not bother providing the definitions again. Here are my impressions from personal experience:

“impressionable”

• This is usually reserved for describing children, when the speaker wishes to emphasize the fact that children are prone to absorb mannerisms or modes of speech from those around them.
• Effectively neutral, more an implication of innocence than anything else, but could be used to snidely insult someone.
• Don’t talk that way around the kindergarten kids. They’re very impressionable at that age.
• “Naïve” is similar as far as indicating innocence, lack of experience, etc., but can be applied to older people more easily. This can be used chidingly, but it’s no more insulting than a potentially unappreciated warning or a patronizing pat on the head.
  • He thinks he’s completely independent now that he has his own car, but he’s so naïve. To make the payments on the car, maintain and gas it up, he’ll have to hold down a full-time job.

“suggestible”

• This means that someone is prone to suggestion. The suggestion could be just about anything, but from what I can tell this word is mainly used with regards to questions of willpower.
• Not particularly negative, but it basically conveys that the person does not have a strong will of their own and that’s usually not a compliment.
• I can’t watch an ad for pizza without immediately ordering some. I’m very suggestible.
• “Susceptible” is very similar, but would only be used in reference to the source of suggestion. e.g.:
  • I can’t watch ads for pizza without ordering some. I’m very susceptible to advertising.

“Gullible” and “credulous” are similar to each other, but the former is far more common. Most of the time it is used to say that someone is easily swindled or fooled by someone attempting to mislead them. This is probably the most insulting one on your list, and it’s not even that bad. “Credulous” just means that someone doesn’t question what they hear, which could make them easily fooled. It’s closer to “impressionable” or “naïve” in that way, but, again, not very commonly used.

Are the above sentences interchangeable?

Definitely not.

I would use "This is not always used." when talking about something that is almost always used, but not always.

I would use "This is not used very often." when talking about something that is almost never used, but is used sometimes.