I think that "talking about" would be better than "speaking about"
It's certainly more natural.
And if I wanted to say 'these' chairs?
What is the full sentence you want to tell?
'mówiąc' vs 'rozmawiać' what is the difference? Is one more formally speaking like giving a speech and the other more about having a conversation?
Just speaking vs having a conversation.
why is tych here genitive
In adjective declension, locative and genitive plural both have -ych.
In locative, do plural nouns all end in ach?
Yes, they do.
https://duome.eu/tips/en/pl#Prepositions (lesson two, bottom table)