Acidic or alkaline water. Which is better?

I have a heavily planted tank with java moss water wisteria Val and some other plants. The fish atm are white cloud minnows and Ballon Molly's. Originally I was told by my lfs to bring my KH up to 5 as it was 3 out of the tap and not worry about ph. My pH is 7.8 now after using alkaline buffer. I went to another fish store and he told me that alkaline water is bad for plants and it's better to be neutral or below. Any thoughts? He told me to bring my akalinity down by adding vinegar during water changes and to not bother with KH and just leave it at 3 as that's what it comes out of the tap.

 

It's nonesense to make yourself dependent of chemicals you have to add regularly as it only costs money and causes fluctuations.
As you have Mollies hard and alkaline is preferred. Tanichthys on the other hand are rather softwater fish. Congrats, this is the typical community tank conundrum.
Considering that hardwater fish (in this case the livebearers) don't do well in soft water but softwater fish can somewhat better deal with hard water:

Add some limestone rocks, they will release minerals into the water so the GH and KH settle at about 10-15° each and pH stays in the 8-ish range. No more adding of chemicals (and I hope you never added that stuff directly to the tank!) and no more spending money on additives for that.

 

It's nonesense to make yourself dependent of chemicals you have to add regularly as it only costs money and causes fluctuations.
As you have Mollies hard and alkaline is preferred. Tanichthys on the other hand are rather softwater fish. Congrats, this is the typical community tank conundrum.
Considering that hardwater fish (in this case the livebearers) don't do well in soft water but softwater fish can somewhat better deal with hard water:

Add some limestone rocks, they will release minerals into the water so the GH and KH settle at about 10-15° each and pH stays in the 8-ish range. No more adding of chemicals (and I hope you never added that stuff directly to the tank!) and no more spending money on additives for that.

I added alkaline buffer to bring my KH up to 5 but that's about it. What about the plants though? His main argument was that plants don't do well in alkaline water.

 

You have some of the least demanding plants, no worries.
The alkaline buffer will have to be added with every waterchange thus you will have to buy it again and again. That's why I advised to add some limestone, because then you don't have to buy more. That's the whole point.

The employee at the store is just trying to sell you stuff you don't need.

 

There are plants adapted to hardwater and do really well in it. Jungle val grew really well in my hard water. Java moss grows okay, not sure about water wisteria. Hygrophila corymbosa grows really well too

A bin of hygrophila corymbosa in 7.8pH hard water:

 

I would stick to 3 dKH and 7.8 pH and see how your plants are doing before taking measures. There's nothing wrong with these numbers for 99,9% of plants in our hobby.

 

Better to select livestock and plants suitable to the water you have than to try to create conditions chemically. The other important thing I’ve found is that PetSmart plants which are sold in small plastic containers in air did terribly, I was spending a fortune. But, got a couple plants from PetsPlus which are sold right out of their aquarium in water. They are thriving very nicely, taking over the tank.
And nerites do better in my pH 8.2 much better than the apples I tried.