General advice / suggestions?

[Kinso]

I am currently building a MOD that essentially adds 2 new item sets, 1 new map and 3 new bosses / minions.
The new set items will require each boss to be killed to craft one item (boss 1 drops an item - boss 2 drops an item that is a recipe with item 1 and boss 3 drops the final item in the recipe)

I just have a couple of questions regarding the whole process:

What would be the best order to tackle the tasks? I currently started with the new set and items.

I know how to create the recipes in the cube, but is it possible to have the outcome as a random item from the set? (One recipe but random set item from a specific set)

I haven't tried map making yet, I assume there's a fair few good tutorials laying around for that, is it possible to have a single map with multiple floors - say 6 floors with a boss every 2 floors? And how much work would be involved in this?

My final question is about the bosses themselves, is it at all possible to control how often x ability is used compared to ability y?

I am also open to suggestions for the MOD itself this is my first large project (I have done smaller things like repeatable cow level, cube recipes and a couple of monster tweaks) but nothing on this scale, so any advice is appreciated.