In my understanding--and I'm no expert on this--but a steady pH is more important than a high or low pH. In other words, most fish can adapt to most pH levels, but sudden swings in pH can harm them.
And, also, I think that mollies are actually one of the few fish who really don't
do well in acidic water, so you might want to avoid them.
But, according to this site, the preferred pH for black skirt tetras is 6 to 7.5, so you're in a good range (that's from Black Skirt Tetra - Gymnocorymbus ternetzi
But that site also says, for the minimum tanks size for black skirts: "Tank Size :
20 gallons (114 liters) - a schooling fish and should be kept in groups of 5 or more."
So, if you get this guy (girl?) a proper school of tetra buddies, you'd be pretty fully stocked in a 10. I only mention it. because you talked about getting a pleco, and I almost did the same thing once, until I found out how big they get. The common pleco can grow to be two feet long.
There are other types of pleco that stay smaller, so just make sure you research which one you buy.
[Edit: lol--my post got ninja'd :) what they said ^]