No to both questions. The only one which might come close is Broken Steel, which expands on the core story a bit. However, it isn't necessary for any core understanding of the main storyline.
The other advantage of Broken Steel is that it does (SPOILER ALERT) allow you to continue playing after the final quest.
According to the Fallout Wiki, knowledge of Fallout 3 is not needed for Fallout: New Vegas, and there are no storyline connections, although like in previous games, there will be the occasional reference to the prior ones in the series.
(I should note that I've not played the original two, and had no issues with 3 and its references to them, so I'm guessing any references from New Vegas to prior ones will be similarly subtle. I do, however, plan on playing the original two!)
Finally, in terms of the DLC, I did purchase, play and enjoy all of them. I was not super-thrilled with Operation: Anchorage, but I thought the others were all exceptionally well-done (even if Mothership Zeta had really nothing to do with the Fallout universe). So naturally your mileage may vary, but it's worth noting that if you want some great additional (and different) stories, the add-ons are, to me, worth the money. Broken Steel is the only one which didn't offer a completely new environment: The others all offered large new lands to explore, with stories to go with them.
According to the Fallout 3 wiki, it sounds like the only ramification is that all other Brotherhood Outcasts will become hostile (except those in Operation Anchorage), and you won't be able to do the repeating quest for them anymore (since the NPC you turn that quest in to is dead).
Best Answer
There is no official lore, at least nothing I can find, that suggests Mirelurks are intelligent or operate on anything beyond animal instinct.
Despite their humanoid appearance, there's no record of them ever attempting to communicate, using tools or building anything- the closest I could find was a single piece of concept art depicting a mirelurk wielding a cudgel. They don't appear to see man-made buildings and tunnels as anything more than caves.
They may have a social structure like many animals do but that does not prove intelligence.
Compare them to super mutants- even the dumber capital wasteland mutants- who can speak simple English, use and reload guns, and build crude forts out of junk.