Can we use British English trends and American English trends (such as spelling, or turns of phrase) in different sentences in the one topic?
It is usually not recommended to mix up different dialects of a language in one text.
If you are describing different characters that speak different dialects, of course, their speech can be in the appropriate dialect.
Apart from that (direct speech), every style guide I have ever seen recommends that first and foremost, you must be consistent in your writing, spelling and style.
So you do not write "capitalization" once, and then "capitalisation".
And if you start mixing up idiom from different dialects in one text, you will only create confusion.