OK - I sure hope your local Aquarium World has good healthy stock. Do you know where they get them from?
I'd strongly suggest you limit your discus to 5 in that size tank, as that's about the maximum recommended as adults for a 55. (and it may not take them long to get there - a few months if they're treated well).
My further recommendation is to leave your tank just as is for now, with only rocks and driftwood, at least for a couple of months, to give your smallest discus a chance to do some growing in a good environment, and you a chance to become familiar with discus traits and behaviors. When you get comfortable keeping/growing out discus, then you could proceed to do some more aquascaping. You won't regret doing that.
Now with the other fish that you have, I think you will be stretching things a bit, bio-load wise, especially if the mollies and the other fish are of any reasonable size right now.
It could be ok, but I feel it's a bit risky on a number of fronts.
The Pleco may need watching, for example, as some have been known to take a liking to discus' slime coats as part of their diet. Not likely, but that could be possible with the catfish too, although I'm not familiar with that particular species.
Your blue lobster could become lunch for larger discus, depending on how small or large it is.
As for the Mollies - if you give the discus the tank cleaning care and attention, that you should, to grow them out well, along with fairly frequent wcs, that may create a water condition environment that is not particularly conducive to Mollies, who prefer brackish water and love to munch on algae.
All of this is not intended to alarm or deter you, just stating things that you should be aware of when keeping expensive discus fish. You don't want to lose them.
3" to 4" discus added to your tank with a 2" er shouldn't be a particular problem, but watch for any bullying by a larger one on that little guy. It would be better if the 4 others you get are around the 3" size.
Those are young juveniles that have some growing out to do, and a not well-developed immune system, so I'm quite sure you would be much better off (less risk) raising those discus without any other fish in the tank - but that's your call.
All the best to you, and don't hesitate to call on me if I can be of any help at any time.
Regards,
Paul