Thanks, key word there is "sound" smart...lol...there are a lot of folks on here with tons more practical experience keeping fish, I just do a lot of research on fish I want, and spend hours every day observing the habits of the ones I do keep....well zoning out in front of the tanks anyways, but I do love my puffers.
As far as the fish listed, I would steer clear of the cudda's, will grow too big for a 50 gal and are quite the predators, the suckerfish could be any number of different fish, but is likely a type of Plecostomus, best to find out what type as some get quite large. Most "Sharks" are carp type fish that have a tendency to either grow large and get aggressive, or are very skittish and delicate (definately not suited to puffer tanks). Eels I don't know much about, but I believe the majority that you find are actually Brackish water fish.
Your best bet would be to find out the scientific name of the puffer in question and then research their needs, you will find that the majority of fish shops have little to no knowledge on the keeping of puffers, and what they do tell you is usually wrong. These forums are likely the best source for info that you will find, as the info comes from folks that actually care for the fish in question.
As this is your first tank make sure you read up on "Cycling" your tank, it could save the life of your fish.
p.s. WELCOME TO THE FORUM