Alright! Congrats and welcome to the hobby. It's addicting and gets even worse when you take the leap into planted tanks. That's where I started.
You have two female guppies. Bad news: females are practically born pregnant. Nearly every female from the LFS
will join your tank full of babies.
Worse news: they don't even have to breed every time they drop a clutch!
Good news: there isn't much room to hide in your tank. So babies will be exposed.
Better news: neons and guppies LOVE to eat fry. I've seen my guppies pop out fry and turn around and eat them.
I admire that you didn't do the trendy thing and buy males. I don't have an answer for your comet issue. If you can return him do it. Other wise.... Sigh. Water changes will need to be a mandatory weekly thing, maybe biweekly depending on your water readings. As you're cycling it's going to be a delicate thing.
Plus 1 for prime! Prime is your best friend.
After the tank runs for several months I say more cories! Preferably the same markings as the ones you already have. I have 6 in my tank 3 like you're, 2 peppered, 1 albino and they pretty much stuck with those who look like them, they'll mingle some times but I rarely see the peppered apart and the false juli stay pretty close too.
As for a center piece I bet a dwarf gourami would be okay.
Once you get the hang of this, come to the dark side and learn about planted aquariums. Good luck.
Sent from my iPhone