I would recommend one of the steel mushrooms with the newer design rather than one of the imported ones. The newer designs have both more surface area and more weight to keep the anchor lying down so that it stays dug in. Other than cost, there really isn't a reason to get one of these.
Regarding the size, if you are planning on a bigger boat soon, 750+ lbs sounds like the way to go. Even if you don't get the bigger boat, you will feel a lot better when a big storm rolls through. For comparison, I use a 500lb anchor on a 30' 10k lbs boat in a mud bottom. The test data for mushroom anchors is all over the place and their holding power is very bottom dependent so it may not be overkill if the bottom is not ideal. Unfortunately, however large your anchor is, it won't help your neighbors boat from dragging and hitting yours. It might be worth asking around and seeing what everyone else is using and how it works in the bigger storms keeping in mind that there haven't been any really major storms in a very long time.
5/8" top chain will require a pretty large mooring ball to support the weight, even in relatively shallow water. Hamilton marine has guidelines for this which should help here.
I have not looked at your harbor regulations but you want to put out as much chain and as long of pendants as you can and still be legal and not swing into any of your neighbors. Many moorings, especially in shallow water, end up with grossly inadequate scope if there is any storm surge. Having long pendants will help increase shock absorption in the system. Maine Sail has done some nice writeups on how to set up your pendants which would definitely be worth a read.
I hope that this helps.