Skipper -- like you, I can't quote the FARs off the top of my head.
I was better when I was actively flying 135.
In short, Commercial and ATP are certificates. Therefore, they apply specifically to category and class -- an ATP for multi-engine land does NOT automatically give you ATP priviledges in a single engine aircraft. A lot of ex-military types have an ATP multi-engine land but only Private for single engine land.
The Instrument Rating is exactly that -- a rating. Instrument ratings are applicable to category ONLY. Therefore, there's just one instrument rating for airplane. You can use it for land or sea planes, if you have those priviliedges. And you don't have to retake a checkride if you get the seaplane rating later
if you earn your instrument rating in a single engine airplane (land or sea), your rating is restricted to Single Engine only, until such time as you demonstrate proficiency in the multi (they don't want you getting your rating in a Cherokee 140 and then jumping into a Cessna 411 and shooting an engine-out missed approach!). If you earn the rating in the twin, you have killed two birds at the same time, because you've gotten the rating and demonstrated the multi proficiency. Of course, you'll almost certain have the joy of shooting a partial panel, single engine approach, but really, it's not that bad (and you have to do it someday!).
It's the same if you get your multi rating in a Centerline Thrust twin (in the GA world, we're talking about the Cessna 336/337, but for military folks, this used to include, for example, the Navy's older T-2 Buckeye trainers). While you will hold the multi rating, it will be restricted to centerline thrust aircraft only until you demonstrate proficiency in a non-centerline twin.
Okay, now that I've totally muddied the waters...I think I'll go now