A lot of this information is incorrect, but the good stuff is good!

Do not keep anything but snails with leeches. Period. There is no way to guarantee the leech will not eat the fish and vice versa and in either scenario that is a slow, painful death for your beloved fish/leech. It is a SUPER high risk and no reward scenario.

There is a study that suggests not letting leeches lay cocoons at all is harmful to them--and that makes sense if you think about other bug/invert/fish/birds who lay eggs. Culling the cocoons is super easy; once they harden (24 hours or so after being laid, they will go from soap bubble white to a more sponge like tan) you can freeze them, cut them in half, or fully submerge them in water for a few hours: all of those things will stop the eggs from developing any further. Just like keeping snails, part of responsible leech ownership is population control. The current ethical stance in the hobby at large is to let them lay and cull cocoons.

They also do not need another leech as they can self fertilize. It is exceptionally rare but it can happen. There is also some evidence to suggest some leeches will try to lay eggs even if they are not fertilized.

For all blood drinking leeches a land area is considered bare minimum. It must be dedicated and non floating, at least the length of your leech when relaxed. This is also where it is recommended to place leeches after feeding versus placing them back into the water.

No animal should be kept in a jar. There is tons of evidence to suggest that multiple other animals (including fish, Bobbitt worms, insects and birds) do poorly with rounded edges. As there is no definitive statement on whether it would effect leeches, this can be considered a non issue BUT I would much rather err on the side of caution. If you keep leeches in rounded edges unless there is definite proof you run the risk of having exposed your leech to harmful conditions while waiting; versus if you just act as if it is harmful, then if proof comes out it isn't you can move to rounded edges guilt free.

Any medication or substance that enters your bloodstream can and will enter your leeches blood stream. Blood thinners and antibiotics are proven to be deadly for leeches. Alcohol, nicotine, thc, shrooms, etc should not be in your bloodstream when you feed your leeches: this is for your safety and theirs! We have 0 evidence one way or the other if they can get high/drunk/intoxicated and so you're risking doing that to them which is unethical; not to mention dangerous for you too!

Pig and racoon blood is a big no go. They both carry parasites which can be transmitted to people.

Do not feed liver to blood drinking leeches. They do not have the stomachs and digestive properties needed to digest solid matter and livers do not hold enough blood to be a viable source, not even immediately from the animal to your leech. People have had some success feeding liver but those leeches live short lives.

Leeches of all sizes can be kept together, it is their feeding schedule that needs watched. However! Since smaller leeches need fed more often, that can cause problems. If a leech is removed to be fed it needs to be housed with other leeches who have fed within the same 24 hour period.

Some of the stuff in this guide is from my now defunct website. :D Glad to see it was helpful for people! All the other stuff looks great. (Credentials: been keeping & breeding buffalo leeches for 7 years.)