Are the 15, 30 & 45 amp Powerpoles interchangeable?

Yes. The 15, 30 & 45 amp contacts all use the same housing. Therefore, they all plug into each other. The only difference between the 15, 30 & 45 amp contacts is the size of the receiving barrel that holds the wire.

Here is a size comparison diagram for the 15, 30, 45 & 75 amp Powerpole Housings

What is the difference between the 15, 30 & 45 amp contacts?

The difference is the size of the receiving barrel that holds the wire.

What is a Powerpole "Set"?

A "Set" includes all parts needed for a single connector. For the 15, 30, 45 & 75 amp powerpoles a set is (1) Red housing, (1) Black housing, (2) contacts and (1) roll pin. For the 50, 175 & 350 amp SB Series sets are (1) colored housing and (2) contacts.

How large are the SB Series powerpoles?

Here is a size comparison diagram for the 50, 120, 175 & 350 amp SB Powerpole Housings.