just a few questions about brackish-marine fish

i love puffers so im converting my 46 gallon tank into a puffer tank
so ive been debating on wether i shuld get a ceylon puffer or a gsp,
i have acses to both fish and i like the ceylon alot more so far because i think it will fill the tank better but i am unsure about something that wuld probably influence my decision alot more if i realy understood it
now i know both fish sort of graduate to saltier water as they get larger but from what i understand the gsp sort of stops at high brackish while the ceylon needs full marine
but i dont realy know what that means other than saltier water

can someone explain exactly what ide need to care for a ceylon vs a gsp

ok so live rock and protien skimmer thanks
anything else you can tell me ive never done marine/very high brackish before

i havent even made a decision yet on gsp vs ceylon

its probably going to be a looooooooooooong time before i buy a puffer at the moment im just waiting for my cherry shrimp and red ramshorns to breed i have had them three weeks an so far i have +0 snails and -3 shrimp

so its not going to well
at the moment i just dumped them in a cycled 10 galon with lots of moss to climb on
so any tips to getting this food to breed wuld be great
also i want to know what gohst shrimp are like in tearms of breading considering $ of 1 cherry shrimp = 10 gohst shrimp

yes there are other food choices but this ones free in the long run so im just waiting for them to get breeding a little bit
ile probably buy a few bags of frozen shrimp or something aswell (any recomendations?)

im not going to start cycling the tank untill im almost ready to buy or have already bought the fish they have the gsps in fresh i still need to check about the ceylons but they are around 1-1.5" so im asuming the store will probably have them in fresh aswell

it is a fully cycled freshwater tank so i wont need to wory about that if the puffers in fresh
and its full of fish that i will sell to the fish store and hopefully pay for the puffer with
they are all freshwater fish so i dont want to add salt just yet