I was planning to make a poison hero but I see nothing about damage over time, even with the poison and fire chart it just a one shot damage amount. Not sure how to make a power without it being to broken. Do I combine the special attack power to like a strike or blast? Also how would I combine this attack with a bite without taking a multi attack penalty and withoug being to OP.
While there are no extant rules for a damage-over-time (DoT) effect, it would be simple enough to come up with something. The real question is whether it's a good idea, but I'll get back to that.
Damage over time could be implemented as a Con to an existing damage power like Blast or Strike. Roll dice to attack as normal, the target defends as normal but instead of inflicting damage for each success on the attack roll, the target takes one counter for each success instead. Then, whenever the target's turn to act comes around, before they do anything else, they have to hand in one counter and take a point of damage.
Inflicting all the damage up front in the normal manner is obviously better, so this is a disadvantage. How much of a disadvantage will depend on exactly how your power works. If there's some simple and obvious action which could stop the DoT, such as pulling out the poison dart you just shot them with, then it seems like a major Con (-2D). If the DoT can't be stopped by normal means, then I'd call it a minor Con (-1D).
As for combining it with another attack like a bite, If I was GM'ing I wouldn't allow it. You're either combining two full-power attacks and effectively inflicting twice as much damage per turn as everyone else (even if half of it's delayed), or if you stick to the dice cap for your combined attack (i.e. the two attacks together don't add up to more than 10 dice in a standard game) then you're paying less for your attack than everyone else (since a 10D attack would cost you 8HD, but two 5D attacks would only cost you 6HD). Overall I'd say that's a big no-no.
There's one other big problem to consider before you go the DoT route at all though. Doing this introduces extra complexity and book-keeping into the game, which is almost never a good thing. I would feel free to suggest something like the above to your GM, but don't take it personally if they decide the whole thing is really too much trouble.
After all, there are other ways within the existing rules to implement poison as a weapon. Fast-acting poisons don't need to be a DoT at all, for one thing. Non-lethal poisons can use one of the status effect powers. Stun works well, or Special Attack if you have something a bit different in mind. Get creative.