If it's a 45 litre I suggest just getting 2 guppies to start with, wait a month and then get 2 more, wait another month and get another 2 and stop when you have 10. Your patience will pay off. Get 2 females first as that way the male won't harrass a single female. Feed the fish sparingly until cycling is complete and then for a week after each new additions to allow all the good bacteria to build up.

Also really think about getting a quarantine tank. I have one that is only 14 litres and I snitched a tiny bit of gravel out of another tank along with a small plastic plant. There is a short heater that is about 50W I think and a cheapie corner filter with floss in it running off a single air pump. That would comfortably hold 2 guppies for up to 4 weeks quarantine and when you've finished adding fish it doubles well as a fry tank.