Aqadvisor says capacity is 86% without the snails, as it doesn't include those.
A few questions - I would like to have some real plants - is there a specific substrate I should look for, and is there a calculator to determine how much I need? Will the snails be enough to control algae in the tank? Should I add an algae eating fish? Especially to get the algae at the top of the tank, seeing as the snails tend to stay on the bottom. What should I use to clean the tank and filter etc with? I don't want to introduce chemicals that will upset the fish.
I would probably have several guppies, not just one as they're pretty small fish to be on their own in a reasonably large tank. Skip the swordtail as these really need a 4ft to do well.
Easy-care plants don't need a specific substrate - you can just pop a root tablet underneath the plants to feed them. Special plant substrates are only needed for heavily planted tanks with plants that are more demanding, and they tend to need extra lighting and CO2 injection in addition.
The best way to deal with algae is to clean it off yourself - an algae-eating fish will create more waste and thus encourage more algae growth, and most don't really eat much algae as they prefer (and need) fish food.
You can clean the tank with a weak bleach solution - dilute about 1:20, and if you have any stubborn limescale marks, white vinegar will deal with them. Rinse well and allow to dry thoroughly before setting up the tank.