either go with 1 peacock or 3.. ime 2 there is no dispersion of the conspecific aggression. You will end up w/ 1 dominate fish and the other 2 will get harassed. personally I'de stick with 1. as for guppies in a 30. go w/ 1/2 males and 3/6 females. They breed fairly mice like. the fry make good food for the peacock and if he can't handle the output I'de toss extras in for the Oscars. Sand would be the best substrate ( pool filter or any NON silica) or a bare bottom, most gravels will scuff them up good and they will be prone to infection. They are much like clown loaches ime, and can get velevet/ick fairly readily, so be prepared. I prefer driftwood and PVC for hideing spots. and Java moss is a favorite of them to hide in ime. They are notorious jumpers, so a tight lid is a must. I use a standard hood, and duct tape the back edges. Platies, swordtails, calmer molly species would also make good tank mates, and larger non-aggressive tetras. Nothing either aggresive or small enough to eat. If the tank is uncycled, start the cycleing process. Add your "tank-mates" and get the "eel" last.
I do keep my 3 fire eels with my Oscar. If your Oscars are a Pair, You will see more aggression then I see w/ mine likely. And he can be a jerk. My Oscar is about 10" atm.. and I do not intend to keep him with my Fires much longer, He's already put some good hits on my fires, they are all about 2x his length. And yes I do see alot of aggression between my eels as well, I'm trying to get a pair. Thus far I have not had to seperate them. I wouldn't risk putting anything but an Adult Tire track in with your heaftys. 1) If you can even find one 2) they tend to be abit more aggressive and stand up better. 3) some solid hideing spots where your Oscars will not be able to get into. barring that.. I'de say your likely to just be offering your Oscars a new toy to play with until they kill it. Particularly if they've been in the tank alone for a long time.
And yes some of the oftenly kept true eels can be alot of fun. but a vast majority are brackish/saltwater. my own personal experiances have been they tend to abit more demanding. But if you want to go SW ever, the true eels are deffinatly worth it.
I'm sure I've repeated myself on some of this. but I'm tired, and it's late.