antiques and modern coexisting  

WoofWoof
I rather like that piece.

According to the seller,...
According to the seller, it's Danish circa 1870.
It's a lovely piece! One of my favorites. Flamboyant, but restrained...if that makes sense. The viewer isn't deluged with lavishness. The presentation, although unintentional, and if it is indeed Danish, fortells the distinct cultural heritage of Danish Modern, or the collective Scandinavian Modern. Just take away the bulbs and knobs...round out the edges.
In a different sense of appreciation...one which I'm unsure if others will agree, but it reminds me in a way of the "machine age". Think of the silent film Metropolis, or even much later, The Jetsons. The bulbs hint at energy...electricity...Jules Verne or more recently Jasper Morrisson. Even Nickola Tesla gets into the frame... I find it so fun to look at.

must it blend?
whitespike, good questions and musings. i like the critical thought representative in your words.
my qualified answer to your "must it blend" question is no. you're free to do whatever you like. and the more sinister, cross-grained, rebellious side of your psyche can be satisfied with the notion that you're also free to do whatever you don't like. [humor intended here to underscore my point!]
to put it another way, you can sprinkle cocoa puffs on your western omelette all morning--and dump blueberry syrup over the whole mess--if that's how you're inclined.
but the rub starts rubbing when you begin evaluating. that discipline alone necessarily employs a hierarchy of criteria against which judgements will be made regarding the absolute or relative goodness, rightness, pleasantness, or whateverness you're aiming to discover.
if all is random and meaningless, then who care? throw all convention, guidelines and values to the four winds and have at it. but to discuss coexistence of dissimilar and disparate objects--along with the "culture" they carry with them--the assumed values are normally those of harmony and a host of other classical aesthetic attributes historically esteemed.
can led zeppelin and frederic chopin coexist? they do in my itunes; but i never listen to them both at once. i certainly could if i so choose, but it will be against informed sensibilities, and i'll experience the "fallout" of my choice. each has their respective place in my musical preferences, and in my estimation, one is not necessarily "better" than the other. their value at a given time is guided by my goals, which, as it relates to music, change more rapidly than my quest for furniture design.
i know, i know...ad nauseum. but you get my point.
and in very brief response to your third tongue-in-cheek question regarding the possible biochemical origin of secret desires, i'd say it's very likely. chemistry is horribly and wonderfully fascinating. at the same time, creative types are intellectually rebellious by nature [and choice], so the urge is one of exploring the tension and testing to see if "the rule" stands up to scrutiny, assault and cross-examination. i wouldn't be particularly worried unless the tendency begins to involve undergarments and chocolate syrup, if you catch my drift.

so called antiques are excellent
COUNTERPOINTS in modern interiors...

It's hard to mix
I would agree with azurechicken. I have loads and loads of traditional antiques available to me and I am trying to mix them into my MCM house but they are kind of fighting each other. I had one success with a 1910 metal chandelier that mimics bamboo. I painted it a flat white and it really looks great hanging down inside the stairwell. It is like an unexpected surprise where you would normally be expecting a sputnik something or other.
I have been editing and editing. I did manage to incorporate a Grecian Lidded Bas Relief vase onto one tablescape but that's only because it's lime green and it blended with the other color schemes in the room.
It's really not easy and you have to have a decorator's eye. I also agree with Olive that Shaker style seems to meld seamlessly. Those round Shaker boxes with the exposed joints look nice with MCM.

My last house was a 3 story...
My last house was a 3 story georgian townhouse in the centre of dublin city,it was an amazing house and my wife and I had it decorated with Georgian,regency and mid-century furniture.
I would say for design you should look at georgian furniture,its more simple than later furniture and nicer proportions.
I think a nice regency or victorian sofa,especially a chesterfield roll back sofa would work well with MCM also try looking at a sofa table or simple mahogany sidetables.