License for using others' code in private plugin?

Hi,

So, I want to create my own plugin. Seeing as there are various plugins in existence, I have a question.
I would like to use their code to write my plugin. By copy-pasta, rewriting, and/or just learning.

However, I do not want to publish my plugin here (closed source, sadly). I plan to accredit all plugins I used in my plugin's code, and when a player types ` /credit ` they can see a list of plugins that I used and referenced.

Is this sufficient? Also, some plugins come with a license in the comments of the plugin's code. I will abide by those licenses; I am asking about the plugins that don't have a license.