You probably aren't going to be able to keep dwarf shrimp without having them eaten by your Dario Dario. Though, Amano Shrimp would be okay.
To save money on substrate, you may be able to get a bag of pool filter sand for under $5. If you want dark, you could get black sand of some sort. You'll get the same results you get with Eco-Complete, which may be a big large-grained for a small tank like that. Unless you're set on Eco-Complete for some reason, I'd suggest shopping around. Maybe even going so far as buying ADA Aquasoil, which would give you nearly everything you'd need as far as plant nutrition goes for the plants you hope to keep.
For the $25 you're spending on liquid fertilizer, you could buy dry ferts and mix them yourself with distilled water to make your own solution. You could do that for way cheaper and will have enough to last a couple years.
AquaClear 20 may give you more options as far as filter media go. Parts are also cheap and easy to find.