In order to use a pluggable transport, is one required to connect through a Tor bridge, or do regular public ingress relays also support transports (such as obfs N) ?
No, you have to use bridges.
While it's possible for relays to run a pluggable transport server and connect them to their ORPort, theres no means to distribute information and what pluggable transports they provide and where. Theres also no real reason that they should either.
So while it's not required it is the only way you'll have some delivery mechanism for the necessary
Bridge lines to use them. There's also no real method for relays to use them in connections between themselves. A Tor relay should be able to make connections to any other relay, uncensored, so pluggable transports would only be a hinderance.