Let's see.
You
do not need to feed it crickets. Most will eat any normal food that floats. Of course, some relish crickets more than anything, and crickets are a good source for live food - but if you are concerned about disease and/or don't want to keep buying them/breeding them, fish food works fine. I feed mine flake food and chopped up frozen krill, mostly (along with blood worms, some freeze dried foods, etc). The krill are his favourite and they are a good meaty food. You'd be surprised how much they are willing to eat when healthy. New specimens are often known to not eat for as much as two weeks when you get them. Just keep trying and be persistent.
YES, YES YES they can jump out. They are *notorious* for jumping out. They can jump over 6 feet vertically. Get a roll of duct tape and cover up any and all gaps in the hood if you have any at all. Trust me. Please.
Don't worry about the pH. Anything that isn't very abnormal (below 6 or above 8.5 ish) will be fine. Basically, go with your natural water and don't mess with the pH. Stable pH is far more important than target pH and trying to change it will only lead to pH fluctuation which is very deadly.
Butterfly fish will eat anything that fits in their mouth (which is very large and built like a trap door) but they have weak jaw muscles and cannot defend themselves well. Do not keep them with any nippy fish or say good bye to that beautiful finnage. Keeping them well fed will help eliminate his desire to eat tankmates...but isn't guaranteed.
Floating plants are, in my opinion, a must. These fish are incredibly nervous critters. Live plants (duckweed, phyllanthus fluitans, water sprite, anachris, najas, anything will work) or even plastic plants will be appreciated. For plastic plants, I just let them float. You can pop off that big plastic piece on the bottom. I also anchor it in one place by attaching it to a suction cup at the surface - this keeps it from going all over the tank.
These fish must not have a strong surface current, for obvious reasons. They need a space with very little movement at least somewhere in the tank.
You didn't mention tank size but these don't really belong in anything under a 29g IMO.
It's real name is Pantodon buchholzi. Here is a brief article I've written up for it:
http://www.myfishtank.net/freshwaterprofiles.php?profile=175
You won't convince me these fish aren't beautiful though
Clickety
That photo is from Fruitbat at MFT.
Do a google search and a search here at AC for butterfly fish/pantodon buchholzi and you'll find a lot of info.
PS - Definitely get the knife fish out of there soon...