you can keep juvies this way, but it won't work long-term, no.
the Oscar and GT would do best in a 75G or larger, but I do believe you could keep the JD and the Firemouth in there for life . . .
the think about the 55G is that is is only 12" wide (on the inside), which is not a lot of room for a full size Oscar or GT, and would be tight for a large JD . . . the 55G is really a best bet for fish that stay under 8"