Will Pikemaster Couplers Couple with Newer Knuckle Couplers?

Most likely should couple, but will the newer cars that did not come with the Pikemaster set operate on Pikemaster curves? AF modified some equipment to make the sharper curves.  

If I do obtain any additional cars, I'll probably stick with the initial All Aboard additional offerings or strictly ones with Pikemaster couplers.

I'm only really looking to add 1 or 2 more cars.  Unless, of course, I can get my hands on a few more panels to expand the initial 6 panel set. 

Yes they will couple fine. The only cars you need to worry about are longer passenger cars and the loco's. If you run them on regulars S track you can get what ever you want. 

Gandy