Is it allowed to copy a text from a website and use that on your own website if you add a reference? or should u always ask that?

You can use texts from another website if and only if its not protected by copyright law. Most of the websites have copyright faqs somewhere in the site like you may or maynot use their texts . But it is advised to ask them other than taking any risk of infringing copyright laws .

No, I dont think anyone will allow you to copy text from their site to your site. But if its not copyrighted then you can ask for permissions.

I believe the prominence of the site, the amount of text copied, the subject content, and the T&C of the site are some important factors you have to take into consideration.

Referencing is always good, even if you modified the text and/or just use it sparingly.

Depends on the type of text. I am sure you can quote from articles on a website and get away with it.
However, if it is text that is specific to that site, for example, description of the website or its services, I would call that stealing.
You don't need to copy stuff. Come up with your own.

Or the threadstarter can also consider hiring someone (me?) to provide copywriting services for the site, haha...

100% original and creative

As a general rule, you'll find trouble if you just copy text wildly from all over the internet. It's always advisable to make contact with the other webmaster to find out what the deal is.

Yes, if the thing is not copyrighted then you can ask for permissions.