A 55 vertical or a 55reg would b fantastic for 5 grays,you could easily do about 9-10 grays as long as you have plenty of greenery and hiding spots for them,grays are aboreal so things like bendy vines,cork tubes etc anything they can climb on thats high will help,they typically love to hide during the day and are pretty hardy when it comes to temps
They are also very hardy with humidity but you dont want a high humidity tank,usually misting once in the morning and once at night and supply a water dish for soaking,but if you did the 55 non vertical you could do a 3/4 land and the rest water,during march/april is when grays breed so if you wanted to breed them a setup w a partial water would benefit the chance of them breeding......any questions pm me i breed grays and am currently setting up some new tanks for them,good luck!