Angelfish is a shoaling species. They live in smallish groups (small compared to the hundreds in groups of characins, cories, etc) and in an aquarium will establish an hierarchy. A group of five is minimum, and this requires a 4-foot length tank with some height. Even this does not always work out; and at some point a pair may well form, and that can bee a serious problem depending upon the temperament of the individual fish.
A "pair" means a male/female that have bonded. Angelfish must select their mates or the "marriage" is unlike to last more than a few weeks if that. If they bond, they will spawn. While some have kept a spawning pair in a 29g, others do not recommend this.
A solitary angelfish might work, or it might not. I do not usually recommend trying this, because I believe in providing fish with what they "expect" rather than forcing them into our thinking. Many aquarists are cruel to their fish without meaning to be or even realizing it.
Cardinal tetras should not be housed with angelfish, though again this can depend upon the individual angelfish. But it is a risk. Rams are another issue; the blue ram like the angelfish must select its mate, plus they need warm water (minimum 80F) so tankmates can be an issue [cardinals would have no problems with this]. The Bolivian is fine with more basic temperatures around 76-78F and although a pair must again be bonded to work, this species does very well as a solitary fish with various compatible characins and catfish.
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