TexasCHLforum.com

I've been happy with the Act-Mag as well. They are OEM manufacturer for some other 1911 magazines.

http://www.act-mag.com/index.php/id-1911-mags.html" onclick="window.open(this.href);return false;

here's a whole forum dedicated to 1911 magazine:
http://forum.m1911.org/forumdisplay.php?f=53" onclick="window.open(this.href);return false;

some magazine options:

http://www.trippresearch.com/store/store_1911.html" onclick="window.open(this.href);return false;
http://shopwilsoncombat.com/Compact-Off ... rtments/3/" onclick="window.open(this.href);return false;
http://www.mec-gar.com/Magazines/1911" onclick="window.open(this.href);return false;
http://store.novaksights.com/1911magazines.aspx" onclick="window.open(this.href);return false;

1. What qualifies as a "quality" brand magazine? Any that I should completely avoid?

Brands to consider: Tripp Research, Wilson Combat, Mec-Gar
avoid: cheap 'gun show GI mags'

2. Will each of the 6, 7 or 8 round officer size magazines fit?

yes.

3. Besides the obvious, what is the difference between 6, 7 and 8 round magazines?

only the obvious: the higher the capacity the longer the magazine. Now with different follower designs you can sometimes gain a round. ie. an 8rd mag the length of a standard 7rd.

4. Would I be better off taking my STI to the store and test fitting actual magazines or are they truly interchangeable?

they are interchangeable, however you may find that the gun prefers one brand or follower design over another