Ok, the Bluegill will need the 55gal tank all to itself, nothing smaller (preferably 75gal) as it has the potential to reach over 12"+ and up to 4lbs though 8-10" and 1-2.5 lbs is about average. Your current filtration will need to be upgraded, rule of thumb is atleast 2-3x's turnover per hour, so the 55gal will idealy need 110-165
gph of filtration just for the sunfish.
The largemouth will grow quickly with abundant livefood and no competition, this fish can get quite large and will need much much more than a 55gal. Like within a year 100gal+, and as adult weighing in at 5-20+ lbs and 2'+ will need atleast
IMO a 200gal minimum.
You could get away with smaller, but would it be fair to the fish?
I know a truckstop in TX with a 800+
gal tank for two Smallmouth and one Largemouth along with various panfish, minnows ect... This is a beautiful biotope tank
Sand is a good substrate and gravel would do just as well, its easier to clean feces/waste off the top of sand but you need to stir the sand regularly to keep anerobic pockets from forming and gravel collects feces/waste so it would need to be cleaned too. If you were to do a live planted tank you could avoid alot of cleaning as the natural cycle takes care of most waste in the substrate.
In a well setup/maintained tank ammonia should not be a problem regardless of what you feed as long as your not overfeeding and your filtration or yourself can cleanup after these messy eaters/poopers.