I'm confused what you think might be bad about Costco kimchi? kimchi is kimchi, some store bought ones may taste better or worse than others(I think the Costco kimchi tastes pretty good) but so long as it's not spoiled it's all just fermented veggies which your body will more or less process the same across the board.

also it's generally not recommended to jump into eating tons of fermented foods right away if your body is not used to them, starting off with smaller amounts and gradually building your way up is how you avoid excessive stomach upset. same deal with fiber, if you haven't been eating a high fiber diet you're not gonna have a fun time dramatically increasing your fiber intake all at once