Looks like you have asked several question here at once. Luckily, the short answer to all of them is yes.
Yes, it is possible to assign a drumset you wish to any song you wish.
Yes, whenever you change to this song, the drumset assigned to it will be automatically loaded.
Yes, you won’t need to do it manually every time.
Yes, you will be able to override the assigned drumset and select any other one you wish.
Yes, you won’t need to select the assigned default drumset again after you switch song to another one, then back to this - it’s default drumset will be loaded for you again.
Yes, BeatBuddy comes with a lot of drumsets, and sometimes songs sound much better via not it’s defaulr drumset - but it is simply a matter of taste.
And, most importantly, Yes, you can create a new song, a new drumset, and assign it to your song!
BeatBuddy is amazing in that it is heavily customizable! This is the thing what most other (even very well built) devices lack.