Well, I think you posed some good questions and its good that you're bothering to ask.
My first question is how far your bulb is from the substrate? 36w over 10g is a decent amount of light. The amount of light actually hitting the substrate is one of those things that can be hard to gauge without actual tests.
That being said, I think you have enough light to start dosing npk and higher amts of trace elements (and co2).
As far as Flourish Comp. and Excel and upping dosage, I would recommend bringing in npk and diy co2.
The only reason I don't recommend the Flourish line is because what you are paying for is the shipping of water, not the nutrients. Its much cheaper/easier to buy dry ferts and either mix your own solution or dry dose the tank directly.
Just to give you an idea of how much more economical it is, 1 - 5ml dose of Flourish Phos. will provide .15ppm (mg/l) in 20g of water. If I want my 36g to get phos. out of that source at say .5ppm, I would have to use 5 capfuls! Thats 25ml per week just for one tank, so in 10weeks I would have used an entire 250ml bottle (about $10-12).
With dry ferts, you just mix your own to the concentration you need and dose. I can add 1tsp kh2po4 to 250ml of water and be adding a little more phos. with a 5ml dose than 25ml of Flourish Phos.
Through Green Leaf Aquariums, http://www.greenleafaquariums.com/aq...ertilizer.html
, you can get 1lb of kh2po4 for $5. At 1 tsp per 250ml, I'll be covered for quite awhile.
If you decide to start measuring dosages more accurately, check out this calculator and play around with it : ) http://calc.petalphile.com/
As far as your question about limits, I would say its another thing thats hard to gauge and this is why I like using the EI method of dosing. I can dose a good amount of each nutrient and at the end of the week, reset it with a large water change.
Currently, I'm dosing about 15ppm no3, 11ppm k, and .5 p in a 29g and 36g, both with diy co2. Switching from the Flourish line has improved growth substantially. And instead of worrying about leaving stuff in the water, the wc after the week's dosing simply removes the excess and resets my nutrient levels.
If you do decide to start adding nutrients, I would definitely add a source of co2 outside the excel as well.
Also, I can safely dose Excel at 3x the recommended dosages without ill effects. Just keep in mind that doing so can get awfully expensive. P.S. you can substitute glutaraldehyde for Excel if you want to cut costs ; )