Um, well this is what the darigan hissi looks like now. I know the pets that were already painted were able to keep the old art, but I don't think that's what you will get if you paint him now.
I think black petpets look pretty good, and I think black plus reptillianish petpets look extra nice. I know you're saving (maybe just planning?) for a maraquan draik, so the cyodrake may be way out of your price range (around 3 mil) but they are probably my favorite evil looking petpet. A plain Cyodrake would look very nice too. And the dofrey isn't reptileish, but I think it works anyway.
I love magtiles with Hissis. It's a shame they only come in two colors..I think either color magtile would look decent with the Darigan Hissi, and if you can get past the fact the the Magtile is about the size of the baby, I think that looks pretty good too.
I think these three also go nicely with both. The green petpets bring out the green in the Darigan's tounge and the Baby;s eyes.
Snowickle is again, pretty dang expensive, but I couldn't not list it. I didn't price check these, but I'd wager that the mortog is next on the expensive list. I listed the Niptor for both because I think it could match either depending on what personality you come up with.