I didn't know that any LFS's carried live bbs as usually it's only adult brine they carry if any.
Frozen foods are no good because dwarfs only survive on live foods.
The newly hatched bbs need to be grown out for an extra day until they reach the second instar stage where they develop a full digestive system and can now be enriched/fed.
It takes two 12 hour stages to enrich the bbs with new water and enrichment material for each of the two stages.
The best enrichment IMO is Dan's Feed from Seahorse Source. It's also the best place to get TRUE captive bred seahorses, dwarfs and standards.
As for the all in one, or plug 'n play, or PNP, they often have a problem in that they don't allow the water to be kept in the range 68° to 74°F that is recommended for seahorse keeping, both dwarfs and standards.
Also, many have a cover over the top that hinders proper air supply for sufficient gas exchange at the water's surface that is essential to maintain a proper pH.
You will probably have to put the filter on a timer so you can turn it off for feeding so as not to filter out the food before they get proper time to feed sufficient.
The size will depend on how many dwarfs you wish to keep.
With my 5g tanks, twenty is not many for that size so for an 8g, you will need a lot more than that, or, waste a lot of food.
You need a high density of food for the dwarfs to feed on because most will NOT search out food like standard seahorses, but rather sit perched awaiting the food to come close enough for them to snick it.
The food then needs to be removed before next feeding so that they only have the new enriched bbs/pods to feed off, and none of the ones that have depleted their enrichment.
I would suggest therefore that you go with the 4g which should have no problem at all with up to 20.
Make sure that the inlet of any filter in the tank, can't suck up the dwarfs, especially any fry they produce.