Black Molly Or Black Guppy?

I want to add one fish to my 10g shrimp and craw tank in a few weeks. My substrate is black with blue pebbles. My plants are almost all green with some with blue accents and one solid blue. My wall(background) is purple. I was thinking of getting one black fish to accent the plants and (match) the substrate. Which one of these will:
1. Do better by itself.
2. Do better in 100% freshwater.
3. Do better in a 10g.
4. Will make the shrimp feel more safe.
5. Do better without a heater.
6. Do better in 7.5-8.5 PH.
7. Do better in ultra hard water.
P.S. Could I get one of each?

 

Mollies need bigger tanks, so go with a guppy.

 

I second what Anders said. Guppies are okay in 10 gal tanks. Mollies need way more room

 

What about a solid black plattie? Do those exist?

 

Solid black sword tails I've seen and actually had....If they do, you might have a problem finding them and they might be a hybrid.

 

Platies can work in this size tank

 

Just wanted to give you all an update. I found someone on aquabid who had true black guppies. So I bout a trio. Wasnt planning on getting more than one but I really liked them. From the pictures and videos they look actaully black not dark blue or dark green! I will post a picture when I get them. They are now individually and as a group the most expencive thing I plan to stock this tank with. So I hope they do well in the tank. This is planning to be a $185 stocked tank! So much for keeping it simple and low budget. Lol.

 

I've seen solid black moscow guppies before, very neat looking guppies!

 

What is the tempature of the tank?

 

@Arades I honestly wasn't planning to have a heater. One came with the kit, so I might actually decide to heat it now that I just bought $17 guppies and plan to buy $7 snowball shrimp and 2 $23 dwarf crayfish. Lol