Well, dont know if I'm the only one with that problem; but I'd love to use the TOS Interiors with another bridge. I mean I like the TOS bridge for nostallgic reasons, but it kind of doesnt fit on my defaint. The Interiors on the other hand fit better to the defaint then the "usual ones", simply because they are better scaled.
So I'd love to use them independent from each other sooner or later...

I didn't know you couldn't. I just figured the Interiors were selected from the same list from which you pick Small Medium and Large.

Are there Small, Medium and Large versions of the TOS interior? (I suppose I should just buy it and find out...)

Nope, there's just the one version and it is inextricably linked to the TOS bridge in its current incarnation.

I'm not exactly sure how "immersion-breaking" it would be for you, but if they were to split out the TOS bridge and interior, you would see things like entering a modern-era turbolift from your Defiant bridge, but when you spawn into the crew/engineering decks, suddenly you're in a TOS turbolift. There's also the issue of location of the mission log node or "ready room". If people were to, say, use the TOS bridge without the TOS interior, you could actually end up with NO ready room area, no area to access logs for mission replay, and also no area to use the ready room computer like at the end of "Mine Enemy".

Another thing has to do with the way ship interior maps are currently set up. Right now, every ship interior map is actually tied to the bridge the player has selected. Each bridge (except the new TOS bridge) actually has every interior (except the TOS interior) included on the map. Selecting the interior in the tailor only sets up which spawn node you go to when "changing decks", and blocks out the portions of the map that you're not using. (You can see every interior on the physical map when you move the cameras around outside the walls when you play back video recorded from the /demorecord utility).

The TOS set is different because that bridge only has one interior set attached to it, instead of three like all the other bridges. They would have to physically add each possible interior to the TOS bridge map if they were to continue in the same style of ship interior design that they're currently using, which could also affect things like performance and lag.