If you feel like one IV isn't enough or you don't want to keep rebreeding, I would suggest getting another Snivy with better IVs. These are the highest IVs you can get for HP Fire:
As you can see from here, you can still get other max IVs and still have HP Fire. The IV set I would highly recommend would be the first one because you get IVs in all your defenses and HP. Besides if you EV train, the Snivy will still be hitting hard despite the VERY small difference (assuming you are using 30 IVs, you only lose 1 stat point). You can get a smaller number than 30 and still possibly have HP Fire.
Hope this helped.
Source: IV Spreads Page and Pokemon Showdown Team Builder to check the stats.