I like fish emulsion, chicken coop droppings, and a fat layer of cedar mulch. Feed the fish emulsion as often as the packaging permits. Use the chicken coop droppings (you can buy them in a bag) every six weeks. And mix the cedar mulch into your ground next spring (and then replenish).

I actually ended up using a store-bought slow release fertilizer by Vigoro that says it lasts for 3-5 months. I hope this wasn't a mistake.

Can I still use the ones you suggested? Will adding too much damage the plants?

Just remember to mulch also.
A nice natural wood mulch.
It will decompse and enrich the soil, worms love it, it will hold moisture in for your plants, and help stop weeds.
The fertilizer you bought is fine.
Next spring before you plant your plants again, buy a couple of bags of compost at your local garden center or Walmart. (cheap)
Use the same fertilizer from this year again, and remember to mulch again.
Mulch is more important than you think. The worms turn it into soil.
Oh, and in the fall, let the leaves rot in your garden over the winter, don't rake them off of the dead plants (if you leave them in the ground over the winter).
Dead leaves rotting in the garden is more effective than cow manure believe it or not.
Good Luck!