Playing FO/Hydra here too, and its absolutly viable in Hell. Doing all kind of MF runs with that build, including pits. Just be aware that you possibly can Solo in 8p games, but it will be very slow going.
From 1-4p games its all fine and you can have lots of fun. My current build atm has a 4k Fireball, 1.6k Hydra and a Frozen Orb with 1pt CM and +12 skills allover, +2 extra for the Fire tree.
Just dont make the mistake of maxing FM instead of Firebolt and Fireball. Masteries always benefit from your +skills, synergies don't. Also masteries and synergies are calculated differently. My build looks like this:
20 Firebolt
20 Fireball
20 Hydra
1-x FM
20 Frozen Orb
1 CM
+12 skills on cold tree
+14 on firetree
Using Tal Rasha Set for the added -xx% resistances bonuses and the added cold damage, and some inbuilt facets and runes.