Regarding TKIY and TCOTDP expansions;
-can I "mix" in the spells, items, Arkham Encounter and ally cards to the
base game without committing to the expansion themes and rules?(or use any
part of the expansions in the base for that matter?)
Im looking to expand the base game a bit without using the full expansions
Or is it all or nothing?
Short answer: No you cannot.
You can mix in the spells, but those ARE slightly themed according to the expansion.
The items are fairly neutral.
The encounteres are HEAVILY themed. Its what makes the two expansions.
Finally the ally cards won't work properly without the encounters.
So in summary - you CAN add parts, but you cannot add parts without comitting to the expansion themes.
As far as rules go they are extremely few new rules introduced with the two small expansions (especially if you ignore the avatars).
PS: You didn't ask, but: Dunwich is FAR more rules heavy, but it is also netral with aspects of the sory IMHO. Its also very easy to only use parts of that expansion like the injuries.