I've done a 20long, 29g and 30g hex all with over 3wpg, dosing Excel for a carbon source. No CO2. I had no problems at all but in the long run it is not cost effective. CO2 is literally plug and play.
Yep.
Keeping algae under control is an issue that involves a lot of factors such as lighting, carbon source (Co2, excel, etc.), nutrients (Ferts, bioload, etc.) and probably other things. I tend to see lighting as the least important because oftentimes we like to over do it.
What plants do you have? It may be as simple as toning down the lighting.
There really is no simple answer as all tanks are different and a lot of times it comes down to experimenting to see what works best.