It says in the Crystal Ball booklet that it was meant to be a single after TMBGITW.

So was it supposed to be on an album? Or did Prince just want to release it as a one-off?

It was on the 1-800-NEW-FUNK album, and Prince wanted to release it as a single
to promote the album. WB wouldn't let him as they'd only agreed to let him release
one single/song independantly (TMBGITW). The fact that Prince had already sent
out promos of the song to radio stations without their consent didn't help matters.

Isn't it considered a single when MTV plays the video over and over? (like Dolphin too)

The definition of a 'single' has become very vague these days, so it's
difficult to say. It all depends on your definition.

I thought Prince had wanted to do a string of singles which weren't necessarily tied to an album, which became (small) part of the overall (very large) beef w/warner bros...

The problem with him and the WB at the time; was Control. Prince is all about control (and rightfully so, its his music) and the WB a big business is surely into controlling their investments. A relationship destined to fail.

amyhr said:

He intended to do a string of indie EP releases, the first being The Beautiful Experience (in fact, they weren't truly EPs, but rather overblown maxi-singles). He also intended to do The Lovesign Experience, The Come Experience, etc., but WB balked at the idea.

JediMaster said:

Yes. Originally the EPs were all gonna be more like mini-albums with 7 different
tracks on each one. WB only licenced him to release one track independantly so
they turned into remixes of the same track. This is also the reason why the b-side
to The Most Beautiful Girl In The World was an alternate dance version of the
a-side (it was supposed to be a proper seperate track). Prince was overflowing
with ideas about releasing all kinds of unusual things around this time.

BorisFishpaw said:

Imagine how cool it would have been if he'd been allowed to do these EPs! Oh man, we woulda shit having all that new music in such a short space of time!

Wasn't an early mix of "18 & Over" originally intended for The Come Experience?

JediMaster said:

Yes it was indeed.
At this time Prince was bored with whole idea of 'albums' and 'singles' and was
looking at creating new ways of putting music out. I've always quite liked this
idea of releasing a series of 7-track mini-albums. by the end of the year we'd
have had at least 3 or 4 'normal' albums worth of material.

BorisFishpaw said:

That woulda kicked so much ass! He should do it now with some of his vault material!