I've read before that it's not recommended to A) do multiple niches in the same blog, or to B) monetize every article I post, why not?
This topic is a little complex
You can perfectly do both and earn money. It's not an issue of ranking itself but a problem with your resources.
When you are little you want to optimize the maximum your resources. This is like stock investing. When you are managing like 1 billion $ you cannot trade intra-day because you cannot have so many good traders that they could made such way of investing profitable ever. But when you are a single stock investor, intra day strategies could be a thing and in fact is one of the main sources of income for many single investors.
Here happens more or less the same. When you a are a solo SEO professional, you tend to micro-manage your resources, and in SEO world, this is called microniching. The wider you open the lesser the possibilities of exploding because your resources are being dilluted. You cannot interlink well you cannot microsite, it's difficult to allocate your linking resources, and it's even difficult to structure your content. All are issues that will cost your time and your efficiency. This is why multiple niches are not recommended when you are soloplaying.
The art of going wide with the fewer niches possible let you run test and pick good feedback over time.
About monetizing posts... why not? If you are going straight for affiliate, it is true that setting affiliates for every single post, tags your page as an affiliate-only-purpose, which, for the eyes of Google is something really bad. Google accept that content creators affiliate, but with quality content. This is why they have introduced the
latest E in TEA. Still if you are going for Google Ads, you can monetize every single page without issue, as long as the ads don't go overly spammy.