Marine salt is used for brackish tanks. Also I don think your geos will like that. Plus Im not so sure a figure 8 puffer and geos is even a good mix. Probably best to either take the puffer back or give it away, or get a separate tank for it.
he is in the tank now. drip acclimated him for 2 hours. he seems ok. the tank has a lot of current, i dont think he likes that. ill bring marine salt and hydrometer to work tommarow. ill raise salt .001 per week .
Two would be fine in a 30g as long as it is two figure 8s together. If you want the best chance of them working together it is best to get them young and roughly the same size. The tank will need to be heavily decorated with broken line of sight which should lessen aggression. Best of luck with him and the other if you get two x