Nope, the compost should have no effect on the fert. and plant, but I do suggest only using 1/2 strength indicated on the fert. bottle that you use, about once every 2 weeks. I didn't realize you had a nice compost mix in there, good on ya! So lightly feed every 14 days or so, and if the plants start to show slight sings of a def., you can always boost it up.

2-3-1 at this stage is good to use, but if you want your plants to give out their full potential, I suggest a higher P number for flowering. If your plants are nice, green and healthy, then why switch it.

My advice is to feed every 10 days, 1/2 strength with the current fert. you have, and switch up to something a bit more powerful and suitable for flowering (high P) after about 2 weeks of flowering growth. It sounds like all your plants are almost at the same stage, so switching food at this stage for vegging would have little effect on the plant.

Hope that made sense. Good luck.