More of an installation question... I haven't played Skyrim in roughly a year and still have my saves. Been itching to do some dragonslaying and try out some of the new mods. Mostly I'm going through Gopher's videos and the Skyrim GEMS site but stumbled across this http://splitkick.com/kickstart-guide-to-skyrim-mods/ . He writes "Decide right now if you’re going to purchase and install any of the official DLC, as doing so later will break a lot of things." How accurate is this? One of my saves is of an Imperial vampire and I intend to purchase Dawnguard sometime in the spring/summer. Is it quite all right to install Dawnguard/Hearthfire into an ongoing game with active mods? Or does Dawnguard add so much functionality that it is more-or-less required for newer mods (a la Tribunal/Bloodmoon with Morrowind)?

The vast majority of mods that didn't play nice with the DLC have been updated by now, it's mostly a non-issue. As far as the DLC breaking things, there isn't any real issue with them. Install the DLC's, unofficial patches, and watch Gopher's vids on "cleaning" mod files with TES5EDIT and you should be good to go.

I wholeheartedly recommend Dawngaurd, it adds a lot of things to the game beyond a nice quest-line. Hearthfire is optional, I enjoy it, but if building a house and "role-playing" aren't really your thing you can skip it.

In all honesty, if you are going to introduce a host of new plugins (official or otherwise) you will always be well served by starting a new game. That's just the way it is.
If I were you though I'd see how far you get with old saves, if you've got them backed up it can't really hurt.
RE: DLC and breakages, Hearthfire is the more likely culprit from what I've heard. It has been reported to make numerous dirty navmesh edits which will result in CTD's if any other mods effect those records. There's probably a lot more to say on the subject but that's what I know about.

*EDIT:

Install your mods using the Mod Organizer rather than Steam or the NMM. That is all.

Which won't make a lick of difference in this case.

My take is that this is specifically regarding the changes that the DLC bring, not loose file overrides.

Pretty much. Official plugins are notoriously dirty, they often introduce some headaches for modders. The primary issues are deleted records (integral ingredient for a guaranteed CTD), and dirty-saves due to scripts being baked into save-game data.

*EDIT: http://www.creationkit.com/Save_File_Notes_(Papyrus) - on the CK wiki, http://www.gamesas.com/topic/1433310-what-is-a-clean-mod-and-a-clean-save/ - of note are http://www.gamesas.com/user/183997-smkviper/'s posts. (http://www.gamesas.com/topic/1433310-what-is-a-clean-mod-and-a-clean-save/page__p__22016386#entry22016386, http://www.gamesas.com/topic/1433310-what-is-a-clean-mod-and-a-clean-save/page__p__22019875#entry22019875)

Installing either of the official DLCs into an existing game is entirely safe. The panic and paranoia over them "breaking things" was largely due to people flailing on about how Bethesda should cater to mods and stop breaking their games. It's exactly the same kind of bogus rhetoric you hear every time a new game patch comes out. Often from exactly the same people, using exactly the same mods they were told were broken to begin with.

Clean the two official DLCs, clean other mods in your load order, you'll be fine. All 3 of my actual playing characters have been through multiple game patches and both DLCs being added along with 70 or so other mods and nothing bad has happened yet.

The real trouble comes when you decide to start trying to yank stuff OUT of your game. That's when it all goes quickly to hell.

Thank you all. Good to know I can add Dawnguard in later. I'm looking forward to it but our wintry weather has me wanting simulated snow and cold instead of running around outside in the real thing! From what I understand a true 'clean save' isn't as simple as it used to be (if at all possible) due to the Papyrus scripts so I can see how uninstalling mods would cause some issues. I generally install using Bash and have yet to try the NMM due to preferring FOMM. What functionality does NMM offer? I don't much care for update detection or integrated download management, and generally prefer to stick with the tools I know. One more technical question before I go off on a spree: Should I ought to be using a bashed patch (Wrye), merged patch (TESEDIT), or both together? I'm used to either one or the other but have found some conflicting information that suggests using both in the case of Skyrim. This seems a bit bonkers to me but Skyrim has its quirks.