I would use a medium and small gravel mixture, such as mostly pea gravel with something penny sized mixed in. My 50 gallon tank has a mostly sand and small gravel bottom, with some larger rocks scattered around. I have mainly java fern in my tank, which won't grow in sand, so I have them attached to a large piece of bogwood. If you have a loose gravel, the java fern should grow well on it, and the variable gravel size will provide hiding places for fry. Some bogwood and flat stone caves might be good to have on the bottom for plants such as java fern and java moss and again for fry to hide under.I would suggest using natural river gravel, which you may be able to buy at a home improvement or gardening store, or even collect from a river yourself (just wash it well either way to remove dust and other undesirable particles). Don't bother buying gravel at a pet store, for it is often coated with a resin, which will cause the gravel to have no buffering capacity, plus it is usually more expensive. Mollies tend to like hard water, so it will be useful to have a substrate that can buffer the pH to keep it from getting too acidic. I'd suggest running the filter for a week or so to remove any leftover gravel dust that doesn't settle, though it's probably not dangerous for the fish (just what I'd do in my own tank). Oh, and watch out if you collect river gravel. If the river is polluted it won't be worth collecting from, and you may get some aquatic insects with it if it's a clean river. They might be interesting to watch and won't harm your adult fish, but some predatory ones may like to snack on fry!

I don't use gravel any more in any of my tanks.
I sprinkle a layer of wet black cow
so you can't see the bottom- maybe 1/2" thick.
Than a sprinkle of 2-3 Tablespoons of bonemeal
and then top it off with 2" of wet sand.
Play sand, construction doesn't matter.
Pack it tight so the cow manure doesn't come up.
Pour treated water in slowly till half full.
Plant your plants and than pour the rest of the water in.
Wait a day or so before adding fish and turning on the filter.
If filter picks up to much sand, cut a course sponge to fit over the take-up head.
I'd also invest in a fish/plant light bulb.
Worth the $20.
You can grow some beautiful plant in these conditions!.....cheryl

I use ecocomplete for my 75 planted tank and I can't believe the way the plants have taken off. It looks very natural now too. You may want to check it out.

Best