lobster risotto -- can i add scallops? didn't have enough lobster

Of course you can add scallops, but I would consider adding crab instead. Similar texture, complimentary taste. In fact, you can consider making two batches - one with the lobster you have, another with whatever you want - and serve smaller portions so your guests have twice as much to taste.

Why not since they have a similar texture and both are delicious.

Sounds delicious! I'd sear the scallops in a hot pan and cook them almost through, then stir them in at the very end so they don't overcook.