First of all, the clams are a baaad idea.
They are pretty much like living rocks. The axies WILL try to eat them. This can cause 2 bad things to happen.
1. They successfully ingest the clam, but can't digest the shell and can't pass it. Impaction.
2. They try to eat the clam while it's open, but the clam snaps shut and traps the axolotls leg, or jaw, or throat, etc. This happens with snails a lot when axoltols try to eat them and they "shut their door", and I've heard of many axies dying from their injuries.
Tankmates that I would consider to be safe would be things like small shrimps (red cherries or ghost shrimp) or small fish like guppies/white cloud minnows. Understand that over time, these tankmates will get eaten. If you do add fish, make sure to observe them to see if they are being too agressive and nipping at the axolotl's gills. If so, you will have to remove them. I've never had this be the case, however.
I've got ghost shrimp in with my axies right now. And I'm trying to breed WCM to occasionally throw in some fry.