So, I have never played proper 40k (yet), but I'm looking to start building my army and I wanted to know what the limitations are on army composition.

From what I understand, the lists build off of unit point values or power score and there doesn't seem to be a real difference between those two other than a personal preference but correct me if I'm wrong on that.

I've seen references to "slots" such as "fast attack slots" and "elite slots" and that makes me wonder if there are rules or quotas for fielding certain units; can you make an army out of a single unit type as long as it meets the point/power requirement?

Could you legally field 2000 pts of basilisks and nothing else, or are the limitations like "for your army you need x amount of troops and x amount of cavalry and x amount of tanks" etc.

Specifically I was looking at some Orks cuz they are right killy and green iz best, and I thought it would be cool as hell to run an army of deffdreads & killa kanz or an army of nothing but shokkjump dragsta's.

TLDR: Could I legally run an army of nothing but deffdreads & killa kanz or would it need to be diverse in composition?

Thanks in advance for saving me having to buy the codex and pour over it for hours lmao