In my opinion I'm afraid there are no sucker fish suitable for a 54 litre tank. Most 'sucker fish' are plecs and even the smallest plecs need a bigger tank. They also make a lot of mess. The smallest sucker fish are otocinclus but I would not recommend those either. They are shoaling fish so you would need at least 6 of them, and your tank is not big enough to grow enough of the right kind of algae to feed otos. They are very picky feeders and a lot won't touch algae wafers.
I used to have a 54 litre tank, which actually held 45 litres of water (the 54 litres quoted by the manufacturer included the air space above the water) so I am aware how small these tanks are.
To be honest, you will soon be fully stocked with guppies. Females can store sperm and give birth for several months without a male. And some of those fry will be male, so the female fry will soon be having babies too. Even if only a few survive, you'll soon have a lot of guppies.