angel w/ oddballs?

I'm not sure about the compatibility, but (IMHO) all of those fish simply won't fit in a 75g together, and most will outgrow a 75 on their own.

Arowanas get very large, and will eat whatever they can fit in their huge mouths. They're too large for a 75g.
Clown knife fish can get several feet long, and hunt by night - say goodbye to the corys. Also too large.
Common plecos can reach 18" and are extremely messy fish - they need a lot of space and heavy duty filtration. You could probably keep one in a 75g with a decent filter.
Spiny eels, I believe, are best kept in a species tank and get quite large. I'm not sure if they're suitable for a 75g or not.

Angels (a group of 5 would be gorgeous) and a group of corys would be great together in a 75 - you could build a gorgeous Amazon biotope...

[edit] Putting any of these fish in a 30g (other than cory cats) is a recipe for disaster. If you want large fish, I suggest you set up a large tank (several hundred gallons), do a lot of research, and then get the fish. [/edit]

The aro and clown knife would not work. Both would outgrow the tank quite fast, and both are pretty aggressive and would probably be intolerant of each other.