You want 75% fire dot; 25% -res Reactive Interface. The fire damage does stack. It can stack up to 4 times* and it shares a stack cap with other players that use the same interface.
*There have been mixed reports of how many times it can actually stack varying from 4-8, but definitely at least 4.
Okay 
thank you so much
For most, it seems to be the damage dots stack 4, the debuffs 8, IIRC.
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Bear in mind almost everyone knows Reactive is the most damaging Interface, and as such in teams of Incarnates, probably half if not more will have Reactive slotted. As such, only so much can stack, so just something to consider, haha.
My Nin/Nin Stalker will be trying out Spectral just to be different when he hits 50, my Crab Spider took Degenerative, and my Elec/Elec Brute took Preemptive to be thematic (and to try to avoid Reactive stacking).
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i believe its actually opposite of this, the debuffs only stack 4x and the DoT stacks up to 8x
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Originally Posted by Rajani Isa
For most, it seems to be the damage dots stack 4, the debuffs 8, IIRC.
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There's also the type of damage done to be considered. It isn't an issue so much in your case since none of the procs does smashing damage, but if you're a fire blaster you probably want to pick something other than reactive so you'll take less of a hit when running into mobs with high fire resistance. And some powers mesh better with certain damage types. Spiders have venom grenadge which reduces the target resistance to most damage types by 20%, but it reduces toxic resistance by 40%. So spiders could possibly get more damage out of degenerative interface than others (I don't have the hard numbers to state it definitively.
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Originally Posted by Ideon
Bear in mind almost everyone knows Reactive is the most damaging Interface, and as such in teams of Incarnates, probably half if not more will have Reactive slotted. As such, only so much can stack, so just something to consider, haha.
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@Inquisitor
from what i know all of the DoTs should do approx the same amount of dmg, reactive will always do slightly more because it is paired with -resist
as Mr Morbid states, it usually is better to pick something that isnt the same as your main dmg, like for example if you picked cognitive interface the psi dmg will be able to bypass most defenses such as enemies unstoppable
Fire/ Energy/ Negative DoT procs all deal the same damage. The Toxic and Psychic procs have a small penalty compared to the others (20%? not sure).
Culex's resistance guide
Just piping in with my normal rant that my characters make these choices based primarily on concept. I don't look at the numbers. Same with my Lore pets.
Of course, sometimes it's nice just to be thematic with your Incarnate choices.
My Elec/Elec/Mu Brute took Preemptive interface, along with Ion judgement and Storm Elemental lore pets just because it was more to do with energy/electricity than the others. 
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Arc: 60092 - Supa Rumble in the Park
"Keep living the dream, and never let any jerk tell you what to do."
-- High-Roller
Im playing SD/SS, from looking at the interface it would seem reactive is the most dmg? but im not sure which one and does the fire dmg stack? =)