I think you only run into the issue where the freehub body rubs on the spokes when mixing 9/10-spd speed parts between road & MTB groups. The 10-spd cog-carrier that holds the largest cogs actually hangs over the end of the freehub body to move the largest cog closer to the spokes to gain extra space for the wider stack. This causes issues when retro-fitted to some older 9-spd parts.

For 7-spd, all Shimano freehubs bodies with 10mm fixing-bolt will work with all Shimano hubs. I won't mention anything about 7-spd DA or previous generation AX parts...