I'd go with a 5g for dwarfs as mine with 20 in it still seems empty.
Dwarfs need live, enriched, ongrown bbs for their food and many find it a pain in the _ss after a while.
You can get the enrichment from seahorsesource.com and it's called Dan's Feed.
If you put those same 20 in a ten gallon tank, you will have to double up the food you put in as dwarfs need a dense field of food for them to survive.
Most don't search out the food but rather hitch on to something and wait for the food to come close enough to snick.
It will be hard to find glass cheap enough to make a tank yourself compared to what you could by a 29g for.