Comparing Parameter Importance Across Models
This may not be the answer you are hoping for (I can't leave a comment because my reputation is too low) but I believe feature importance in Sci-Kit Learn is derived from the average depth at which each feature appears. I don't know of any built-in functions in Sci-Kit Learn but perhaps you could scale the feature importance using the depth of the tree and the number of features?

This article may help with developing a comparative measure of feature importance: https://medium.com/the-artificial-impostor/feature-importance-measures-for-tree-models-part-i-47f187c1a2c3.