As far as I can tell, none of the runes for Sacrifice causes the explosions to deal Fire damage - I think you should assume it does the base type of damage unless otherwise stated.
There is, however, a rune for Zombie Dogs which sets them on fire...that one, I believe is a constant, small bit of fire damage on top of their regular physical damage attacks.
Since you already have Firebomb as a form of spammable AoE, you may want to consider something like Haunt runed with Resentful Spirit as a way of rapid-killing single targets. Resentful Spirit lasts around 2 seconds, so you should be able to get a couple Firebombs off while it's ticking.
Ah yes it was ZD that I meant, sorry, exhausted^^
Thanks for the suggestions
The problem with Haunt is that I'd prefer something poison related as it leaves me with 3 different types of damage; Fire, physical and poison. I did consider poison dart, but due to animation speed, the DPS is kinda low. I'm thinking maybe zombie charger, I believe if I just click it on a monster, the zombie will hit it no problem. Think it goes out in a straight line.
Edit: I just realised it's arcane. I really need to sleep...
Rain of Toads is an AoE spell which does damage over time over a small area. The skill appears over the area you click.
Since Ghost Bomb is already a cheap AoE skill, it would be more balanced if you were to take an expensive, damaging close range spell, for those times when enemies close in on you.
Consider the following close-range poison damage alternatives:
Plague Bats - least mana, least damage
Kiss of Death - most expensive, well balanced
Zombie Bears - most damage, least accurate, least range
If you would like a skill which is more in line with the functionality of Rain of Toads, you should consider these long range poison damage alternatives:
Corpse Bomb - small area, most resembles Rain of Toads
Slow Burn - damage over time
Lob Blob Bomb - damage over time
Finally, you can add Unstable Form to mix things up. This will cement your build as a defensive build, but also give you the ability to really pack on the single target damage when used in conjunction with Paranoia. It will be trickier to use than Rain of Toads, but since it has multiple purposes, I can't see it not fitting in your build. The Hex status plus area of effect damage makes it an odd but attractive option.
Personally, I nominate Kiss of Death, because it is a highly damaging skill which can deal direct damage as well as damage over time. The range is short to medium, so you do get a bit of breathing room. It is also very expensive, making it suitable as a skill to rid yourself of all that excess mana.