Grymatta,
Check out
www.applesnail.net, as it will have all you need to know about Mystery snails.
I had two golden mystery snails which, according to what I've read (and as Lotus stated), don't really eat plants. However, in my case they did munch on the banana parts of the banana plants.
Folks say that they will only really eat dead/dying plants, however mine seem to be growing (although the chewed up banana parts eventually fell off, and new ones grew back in place of them).
I wanted to get the plants growing though, so I ended up giving my mystery snails to an LFS.
I really enjoyed having them though, and I'm thinking I might get some of the black ones...or maybe a bit of both the blacks and the goldens...
Seems like the plants will grow back anyway.
As for stocking a 3 gallon Eclipse. I've got close to a 3 inch blue cray in there (this is not a permanent home for him...he will get too big), along with 4 cherry barbs. I actually had 5 in there to begin with, but one of the barbs became bloated and sickly (but not due to water quality...I test very regularly, and have not had any water-quality issues to speak of), so the cray ate it.
Other than that, everything has been just fine.
Whether or not a tank is actually "over-stocked" is primarily dependent upon your level of expertise...the care, and consistency with which you're willing to keep up with tank maintenance...and the actual 'swimming room' that the fish have.
I feel that it is cruel to keep fish in an environment that keeps them from swimming around "naturally" (or as 'natural' as we can get in our little man-made fish tanks), with enough swimming space. That is why I refuse to put zebra danios in either of my tanks, as I feel that even the 12 gallon (which is on the tall side) doesn't have enough horizontal swimming space for them.
The cherry barbs however, even when in the 12 gallon tank, were relatively 'inactive' compared to danios (and compared to my other fish as well), which is why I moved them to the 3 gallon tank. They seem to be doing just fine, and I have no problems whatsoever with the cray eating them if he so chooses.
I'm babbling on, so I'll stop. Just wanted to share my opinions on these topics.
Big Vine