Can swordtails live with upside down catfish?

Verdict
Yes

Yes. Swordtails and upside down catfish get along in the same tank. Both are fine at 72–79°F. Plan on at least 30 gal.

SwordtailSwordtail72–79°F · pH 7–8.3 Upside down catfishUpside down catfish72–79°F · pH 6–7.5

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 72–79°Swordtail 72–79°Upside down catfish 72–79°
The green band is the temperature range both can live in.
456789Both: 7–7.5Swordtail 7–8.3Upside down catfish 6–7.5
The green band is the pH both tolerate.

Size to scale

Swordtail · 5 inUpside down catfish · 4 in0"1"2"3"4"5"6"
Adult length in an aquarium. A fish can swallow anything that fits in its mouth, roughly a third to half its own length for predators.

Care at a glance

SwordtailUpside down catfish
Temperature72–79 °F72–79 °F
pH7–8.36–7.5
Adult size5.0 in4.0 in
Minimum tank30 gal30 gal
TemperamentSemi-aggressivePeaceful
Swims atMiddleBottom
GroupGroups of 3+Groups of 3+
DietOmnivoreOmnivore

Better tank mates for swordtails

All swordtail tank mates

Better tank mates for upside down catfish

All upside down catfish tank mates

Questions

Can swordtails live with upside down catfish?

Yes. Swordtails and upside down catfish get along in the same tank. Both are fine at 72–79°F. Plan on at least 30 gal.

What temperature suits both swordtails and upside down catfish?

72–79 °F. Swordtails need 72–79 °F, upside down catfish need 72–79 °F.

What size tank do swordtails and upside down catfish need together?

At least 30 gal, set by the swordtail. Add space for each extra fish.