I've just set up a 40-litre tank to grow hornwort & duckweed.
It got a simple sponge filter (the one that use air pump) which I hope is enuff to support a few guppies.
I'm thinking of getting a male betta or a few female bettas in with the guppies, and wondering if they're ok together.

& what are some stuff that are essential for keeping betta?

cheers

As far as I know bettas and guppies wouldn't do so well together. Guppies have pretty fins that sometimes kinda of resembles a betta. Betta's are, in my opinion, very vain creatures and don't like anything that might be as pretty or prettier than they are. I think that the betta would attack the guppies and that just wouldn't end well.

Hmm what if the guppies are just plain ones, without the fancy tails?

i think they go after the fins no matter what they look like. when i had got my first guppies two had died so i just had one so i stuck a betta in there to try it out and he kept flaring at her, never attacked her, but i took him out before he could. and my female i had in there was plain looking, she was a very pale yellowish peach color, almost no color at all.

Small schooling tetras or cories might be better if you REALLY want other fish in there. Tetras need to be in a school of at least 6, especially if they are with a betta. How many gallons is 40 liters?

Isn't 40L like 10ish gallons?

And no, I wouldn't put bettas and guppy together.