sexing them at the lfs was a failure for me. they are usually .5" or less, just too hard.
let me try to explain my thoughts.
30 tetras is much more impressive than 5. They school and look great as a big group.
dp are more like plecos. having a bunch in a tank isn't really any more enjoyable than having 1 or 2.
since you can't really keep most fish with them it seems sort of a waste to dedicate a bigger tank to them when, personally, I get the same happiness from seeing a few as many. (unless you are talking about a really big tank, maybe a 100 gallon long with 40 dps in it, then it takes on a new dimension just for novelty's sake)
I really like the dimensions of a 20Long, so a 20L with a 2-3F and 1 M is what I would do.
A 10 or 30 would work but I don't like the dimensions as much.
You could always put a dozen in a 30 gallon and then just keep the ones you want, they grow fast and in 3 months at most you should be able to sex them easily enough.
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry