I'd stick with the one 24/7 then and see how it goes. Maybe favor the light a tad towards the front side of the tank if you aren't observing very good growth with HG and see if it helps. Other than that, the finnex should be able to grow most plants without any issues assuming CO2 and nutrient levels (Macros and micros) are in check.
HG does well with CO2. Bigger name brands like GLA, CO2Art, and Aquatek are all good starting points for regs. I am sure you can find people on this forum whom can also make good quality regulators for a cheaper price than the big name vendors. Just remember, everyone has their own good and bad experiences with regs. Just because one person didn't have a great experience with it, don't count it out. Do your research. For a 55 gallon I would look into getting a 5 to 10lb CO2 tank to minimize refills.
In my opinion, I'd start looking into dosing dry ferts. GLA has a great E.I. fert kit. Saves money in the long run compared to liquid ferts.