Hello, thank you for using Astah.

Currently there is no option to make some room in Sequence diagrams when you try to insert some models I am afraid. So you will have to move models to make space manually.

You are able to move multiple models (Interactions) all once by selecting the range of diagram by left-drag. Also [Map] view (One of the views of top left pane of Astah window) may help you to show certain area of diagrams in Diagram Editor.

We have received much feedback on sequence diagrams to make it easier to modify existing ones and it is one of items for us to improve in the future.

If this does not meet what you asked for, please let me know.
Thank you.

Satomi