Thusly "Jan" Spake Unto All:

One article on the net says hydras are no concern for livebearer fry - is
this true? If not, how do I get rid of them?

IMO hydras are no concern for pretty much any fry. I had literally
tens of thousands of hydras in a 10 gallon tank I raised lampeye,
killi, and rainbowfish fry in.
The biggest problem with hydras is that they steal food from the fish,
and eventually I tired of them. I raised the salinity to about 3 ppt
(that's 3 grams of salt per liter water, or about one large, heaping,
tablespoon of salt to 2 gallons). At first it seemed nothing happened,
but after two weeks the hydras were gone.

Guppy fry should not have any problems with 3 ppt salinity if you wish
to try, although some plants and apple snails wont like it.

There are other ways of dealing with hydra, but that usually means
using toxic chemicals like formalin, or medication like fenbendazol.