Question about planted aquarium and fishless cycling

Thor or others,
Got an issue I could use some help with. But let me catch you up.
All the guppies I bought from the pet store ended up dying. The last to dye had it's scales kinda raised one other did as well, so I think maybe it was dropsy. But one or more of them delivered and I had 2 fry that survived and are now about 10-12 weeks old. Lost one of my shrimp but the other has gotten big. My pleco is getting big and does creat a lot of waist. I may start trying to find another home for him. I upgraded my light to a 36in quad T5 about a week and a half ago and thought my plants were doing better. But they are still getting brown spots on them. Initially the brown started at the tips but now more spots throughout. I had been leaving the old light on too long because I didn't think the plants were getting enough light. I ended up getting a lot of algae. Thought maybe the wrong type of light was causing the abundance of algae but since getting the quad T5 the algae is even worse. My water temp is at 78 F and I am leaving the light on 12 hours per day. I just added 3 albino cory, 2 mollies, & 4 ballon mollies. The fish have been in only 24 hours but seem to be doing very well. I need to increase the number of guppies but the store had their female guppies quarantined. My two little female guppies are hiding out a lot since they got new tank mates.

Any help you can give me on the algae problem would be appreciated. Also, is it time to use some plant food? If so what do you recommend. How about CO2? I still would like to add more plants but not until the ones I have start looking better. I have been trying to get rid of the snails as suggested but still have some. Do they arm the plants?