Ethernet Shield

I’m new to working with PCB; so new that I’ve honestly not done any pcb work before. But it also seems like something fun and worth trying. I have “a ton” of questions but I know how the internet works and if you ask them all at one time the information you get back is pretty garbage and useless so instead I’m asking only two questions:

Are PCB of the Arduino form factor generally pretty compatible? (I.E. Does one Arduino form factor PCB generally work with a shield from anywhere or are compatibility issues common)

My second question is what is the cheapest board/Ethernet Shield combo that you can assign an IP to?