Hi and welcome to the forum
If you want to breed guppies, just have a group of females in a well planted tank. In 6-12 months they will have used up the sperm packets they got when living with males and then you can add a male for a few weeks to fertilise them all. Then remove the male and leave the females to carry on breeding.
Guppies should be dewormed and treated for gill flukes before adding them to an established tank. If you don't have a quarantine tank, treat the main tank for worms and gill flukes when you have the fish.
Guppies do best in water with a
GH around 200ppm and a pH above 7.0.
Bettas, suckermouth catfish and khuli loaches come from softer water with a
GH below 150ppm. Bettas and plecos also come from water with a pH below 7.0.
You should contact your water company and find out what the
GH,
KH and pH are.
Otocinclus catfish should not go into new tanks. They need algae, biofilm and driftwood. It takes several months to get a decent layer of biofilm in the tank.
As to whether you should get Khuli loaches or Otocinclus catfish, which do you prefer?
You will probably see the Otocinclus more and they will help clean up algae, whereas the Khulis will hide all day and do nothing to algae.