Wow...overdosing diy co2 on a 55g? What kind of setup do you have? I would love to have that problem.

If you're sticking with diy, get a drop checker, it's really the only way to do diy successfully since you will know right away when the co2 level starts to get low.
I'm not really sure how to answer your specific questions as I only use individual powdered fertilizers, but you will need to be dosing nitrate, potassium, phosphate, iron and traces; and calcium and magnesium too if you have a low GH (or GH booster). With high light and co2 it's not likely you will get away with not dosing any one of those things for any length of time, although with as few plants as you have in the tank right now you might be ok for a little while until they use up their reserves plus whatever is in the tank for whatever you're not dosing.
I also don't know much about metal halide bulbs...but the general idea is that for a co2 tank with fluorescent bulbs you want around 3 wpg, 4wpg maximum. Any more than that, and you're just growing extra algae, usually.