I tried a search and couldn't find much relevant, so I hope I'm not just rehashing old ground here, but I came across the Atomic Zombie website and was thinking about trying to build a tadpole or even a tandem with plans from them... Has anyone here tried any of their plans? Are they worthwhile? Is it really something that a person with little or no experience can handle?

bikes like that are discussed in the Alt bike culture forum. Atomic Zombie gets a mention over there fairly often

I have a sneaking suspicion that you'd be much better off simply hacking some old bike frames into a tandem or recumbent. Much easier, and a lot of the hard parts are done for you.

I looked in that forum and it seems pretty deserted. Though I didn't read every post from the last year, there are only 3 pages of them and I found very little to go on. Though the gallery on the atomic zombie site has a bunch of weird stuff, the plans include some pretty normal looking bikes.

Maybe... but the bikes they show on the plans page seem to be hacked together from old bike frames and some extra bits of metal... it would just be nice to have a plan to follow since I've never done anything like this before.

If Atomic Zombie isn't the right source for plans, is there a source anyone can recommend?

If you are more interested in recumbents, there are forums that have discussions about construction. You could check in the bikeforums recumbent forum.

Have you checked out fleet trikes?