Did any other prime also have the ability to transform or just Amalgamous?
Dear Transformation Tracker,
Certainly. Amalgamous was just the best at it.
Just because each of the Firstforged had specialities, didn't mean the others were inept at it. I may be the Guardian of Time and Space, but Onyx and Alchemist had their own abilities to see into other times and places. Solus was the paramount creator, but we are able to make our own tools when she is unavailable; Alpha Secundus boasted his ability to make computer systems outweighed her own, while Nexus always liked making his own blades. Autonomous Maximus was the ultimate warrior, but no one could deny that the Fallen or Beta were among the greatest masters of battle.
Likewise, while Amalgamous was a master of transformation, each of the Thirteen had one or more alternate modes we can use, depending on our bodies at the time. You are well aware of my preferred Solar Sailor mode, and Primon was incredibly fond of his Laputan Machine form.