Yes, a 5g works well for dwarfs and I've had up to around 50 in a 5g.
However, for only 6 dwarfs you will waste a lot of enriched brine shrimp nauplii as you will need pretty well the same density of brine for 6 as you would for 25 or so.
If you could start off with a smaller tank first and as the numbers increase, then change to the 5g, it might work out for the best.
Most dwarfs don't hunt their food down, but rather sit perched waiting for it to pass by close enough for them to snick it up. This means you need a fairly dense feeding of the nauplii to get sufficient food to each of the dwarfs.
Then, you need to remove uneaten nauplii before the next feeding so that the dwarfs can only feed on the newly enriched nauplii as the previous ones will have quickly dissipated their enrichment level.
