Hermione’s reasoning behind the Galleons:(emphasis mine)
“Well - I thought it was a good idea,” she said uncertainly, “I mean, even if Umbridge asked us
to turn out our pockets, there’s nothing fishy about carrying a Galleon, is there?
Hermione only expects a single Galleon for most of the DA members. It is probable that some could afford more than one, but apparently not the majority.
Then Harry actually brings up(sort of) your point in the book:
suppose the only danger with these is that we might accidentally spend them.”
and Ron gives the answer:
“Fat chance,” said Ron, who was examining his own fake Galleon with a slightly mournful air,
“I haven’t got any real Galleons to confuse it with.”
I think the idea for the majority of the DA members would be that they would have fewer Galleons than the other coins, and would therefore be less likely to mistake it for a real one. As Hermione suggests that carrying a single Galleon would not be fishy.
This is further evidenced according to this Muggle-Wizard Currency Calculator, where a Galleon is worth approximately 10 U.S. Dollars or 5 U.K. Pounds. To a teenager, not a suspiciously large amount to be walking around with, yet large enough perhaps so that they wouldn't be carrying too many around (so that it couldn't be mistaken for actual money).
Furthermore a Galleon really is the only currency she could use realistically, as Sickles and Knuts were too inexpensive and therefore common for people to be expected to keep them separate (59 cents and 0.29 UK Pounds for the Sickle) and (2 cents and 0.01 UK Pounds for the Knut).