Everyone in this tank is flaring a little!
I've mentioned this before and haven't gotten any feedback, so ICBWAT, but ISTM that all corydoras are not created equal! I only have experience with two types, the peppered and the bronze, but they are like night and day, and I bought them all at different times.
The peppered are quite active (when the light is on and after feeding time, when all the fish get hopping), not at all shy, and seem pretty easy going. The bronze are skittish and tend to freak out over everything. I mean they look like they are having a heart attack. They are the last to come out and eat and the first to go back in the plants and hide. Do I just have weird fish? Has anyone else noticed that their bronze corys are high strung? I have a pretty non-threatening environment here and they are still really jumpy.
Anyway, I guess what I am saying is, it might not help Kevin a lot to get more bronze corys if they are all going to be like the one he has now, just as a group instead of alone. He might be better off trying to trade the bronze for something he perceives as more active or less skittish, keeping in mind he hasn't much space. My krib is pretty aggressive, and my peppered cory wasn't fazed by him at all when they shared tank space in a ten gallon.
Kevin, what I am hearing is that you'd like to keep your shark, at least until it gets too big or you can get a bigger tank for it, but you are hoping for some advice on how to give your other fish a modicum of security. I find that having a lot of (fake, in my case) plants toward the back of the aquarium gives my shyest fish plenty of security and allows everyone to sort out their designated areas.
Having another little cave might be good, too, if you have enough space. I find that my bronze corys do hide in the plants or in the corners of the tanks, but my peppered corys just rest anywhere at the bottom when the tank isn't lively.
As far as the guppies go, I can appreciate that you want something more colorful to look at. and the fancy male guppies do have those beautiful tails. If you had plenty of plants, it might work out with the guppies, or you might consider just adding a couple more platies, since you have some of those already. The stores always seem to have very colorful platies and swords. The platies might have a better chance against the shark, if push came to shove, since they're bigger. Just try not to overstock.