I would advise against it. I have experience with Oregon Red-Spotted Garters and I have found them to be extremely aggressive at feeding time. They are best kept separately but they can be kept together if necessary but must be fed separately. A 75 gal tank is considered way to large for small garters. Feeding them is difficult in such a large enclosure. Your best bet is to house them separately in small enclosures. If the Checkereds are still on fish you can house them together until they get larger. Another problem with group housing is that it makes for poor husbandry. It will be imposable to monitor what each snake is eating, how much they are shedding, and other notes needed to track health and well being. If an individual snake develops a contagious problem like parasites it will quickly infect all of them. Smaller enclosures for individual or 2 or 3 of the same species is the best method for garters and most other snakes. Some Garters must be kept alone but I do not believe this is true of Checkereds. 5 gal or shoe box size is good for individual babies or 2 to 3 neonates. 10 gal or a sweater box size tub is a good size for baby garters. You want to keep the space manageable without overcrowding.
Randy