Are there any standard rules for choosing between greater than and more than?

I assume you're talking about grammar rules? I'll move this to the general verbal section - make sure to post in the appropriate section next time.

"greater than" is used for uncountable nouns. "more than" is typically used for countable nouns (though you can also say something like "She likes Sam more than Amy" - but that construction doesn't show up much on the test).

So: "The population density of calico cats in San Francisco is greater than three per square mile" (here, the word we're describing is "density," which is not a countable noun - you wouldn't say "1 density, 2 densities, 3 densities...")

vs: "I have more calico cats living in my home than you do." (here, we're describing "cats," which is a countable noun because you would say "1 cat, 2 cats, 3 cats...")

Stacey,
Then how come it is used differently on this question
http://www.manhattangmat.com/forums/post6511.html

Thanks for your help.

Please clarify

the best rule i've found is to test the legitimacy of placing "greater" BEFORE the noun (i.e., as an adjective), vs. the legitimacy of placing "more" BEFORE the noun (i.e., as an adjective). if it works as an adjective, then it should also work in the construction you're describing here.

in this problem, which i'm sure is the problem that prompted your query in the first place, you can clear up the issue of "numbers are greater than..." vs. "numbers are more than..." by doing this inversion:

the gyrfalcon has greater numbers --> this is ok
the gyrfalcon has more numbers --> this doesn't make sense

therefore, "greater" is the preferred version.

try this if you ever run into this issue again; it will almost certainly work.