So as I understand it an " Action: " ability can only be used once per round unless otherwise stated, however its unclear as to whether that " Action: " ability on other copies of that card including ones from your opponent's decks cannot be used after the " Action: " ability is used. Is there any rules information that can clear that up?

Any given triggered ability can be used once per round. This is a limit per player, and per copy of the card. If you have two Shiba Yojimbo, you can interrupt two triggered abilities targeting shugenja you control.

There are abilities that have separate 'limit' restrictions. These replace the normal limit of once-per-round for that ability, such as Spyglass's 'Limit twice per round.'

There are also abilities that have 'Max' restrictions, such as 'Max 1 per round.' Max is a different sort of 'hard cap' on the number of times an ability can be used. Unlike normal and 'limit' restrictions, 'Max' restricts all copies of that ability for one player. This means if you have two of the same character with a Max 1 per round ability, you can only activate one of those two abilities, even though you have two instances of the ability. Similarly, Way of the Dragon will not increase the number of activations you get above a Max restriction. So you can't Way of the Dragon Togashi Yokuni and copy two characters' abilities, because his ability to copy is a Max 1 per Round. Max behaves differently than normal rules in this way.

Note that 'max,' 'limit,' and the normal 1-per-round restriction are all player-specific. If you and your opponent have the same character in play, both of you can use that character's triggered ability. If you gain control of an opponent's character that's already used its ability, you can still use its ability yourself. This even includes abilities that are 'Max 1 per round.'

So, easy answer to your question: No, the once-per-round limit does not extend to other copies of that card in yours or your opponent's decks UNLESS it has a 'max' restriction, in which case it does extend to your other cards but not your opponent's.

RRG page 10-11 - check out the sections for Limit x per [period] and Max x per [period].

Its essentially as Ardon detailed, but that's where it is in the rules.