I have taken Lucy over to my Mom's house several times to play with her corgi/sheltie mix and they get along great!  Play together, rest together, and even eliminate together.

Lucy also has several dog friends here at our condo.  She met a shih-tzu puppy a few weeks ago and they played just fine.  We were walking the other night and Lucy and Gucci played well then too.  BUT last night, we took Lucy out (since we are in a condo, what initializes our adventure outside is Lucy's need to go to the bathroom - we always avoid the other dogs so she can potty in peace, then a little playtime is ok) and walked opposite of Gucci to use the bathroom.  Well I'll be darned if the human and Gucci came down to where we were.  Lucy had already tinkled, but no poo yet.  The neighbor is very friendly, but does not keep Gucci on a leash ("it is torture".  Lucy spent her time trying to get away from Gucci and he kept following her.

I was watching them, but at times, I was engaged in conversation with the neighbor and my boyfriend.  I heard a little "grrr".  Then it happened again and got my attention.  Apparently Lucy didn't was finished with Gucci and he wasn't understanding her, so Lucy would sharply turn her head and give a little warning growl.  SHE HAS NEVER DONE ANYTHING LIKE THIS BEFORE!  I don't want it to progress to something worse. 

My Mom suggested to avoid the situation, but I don't know if that is entirely possible in such a small community with few green patches.  Any ideas on how to curb this behavior?  I am also planning on speaking with the Board here about sending her a reminder about the leash law.

That actually might NOT be "aggression"---it could simply be her way of telling Gucci "back off, Dude, I've had  entirely too much of you in my face!"  My dogs do that to each other, and they learn to respect each other's space because of it.  Just watch her body-language next time and see if she's really being aggressive, or just telling him "that's enough!"  If that's all it is, then she'll speak her piece, and get over it and go on like nothing happened.  If it's truly aggression building up, she will want to keep it going and may show signs of getting tense around Gucci..

I think you will find that as a dog grows from a puppy, the personality changes. When a pup, they are so accepting of dogs, people... and as they get older they become more wary, more cautious... and the typical "doggie behavior" will surface as the dog becomes what his/her inherent nature intends for him/her to become. This is another reason why imprinting certain behavior AT AN EARLY AGE is SO vital !! It's so much more difficult to change a behavior once it is engrained in a dog. It's easier to recognize the "signs" at an early age and correct what is correctable. Regardless of the amount of training... dogs simply become more cautious and more territorial as they get older. People do too !!! We become less accepting of everything and everybody... right?