Yes, you can grow 6 plants in the SH tub, but it'll get crowded as the plants get older.Why? Bubbleponics can't grow more than 3 plants? Or are you just saying that specific system can't? Isn't bubbleponics really DWC with hoses anyway?
I start 6 plants in the SH system and once the roots start to grow out of the nets, I'll then transfer them into another system for the balance of the plants life.
Yes, SH is a DWC with feeding tubes to help the seedlings to establish themselves. Once the roots are in the water(a decent amount) and can then remove the water pump & feeding tubes. Having at least two airstones will also be beneficial.
If you want to grow in the SH system from start to finish, I recommend 3 plants max due to the plants needed room to grow.
But I guess if you were growing some dwarf plants, 6 may work out nicely. But I'm not sure.