changing the table structure (which is what you are doing adding a new page type) is going to take a while to run, but this is only an issue when adding new code to the site. It should not affect day to day operations.
If you do want to remove some of the rows from the Sitetree_versions table, you could run an sql command like:
"DELETE FROM `SiteTree_versions` WHERE `Created` < '2009-03-01 00:00:00'"
This would delete all rows created before March 1st. You could write something trickier to only leave the two most recent versions, but that would take a bit of effort....
Deleting rows should not affect the revision function (other than removing old data).
I think I'll remove the old revisions by date. The other method will take too long to implement.
Having too many records in the versions table is quite dangerous since running /dev/build on a live server can kill the performance on a server.
Perhaps it is worth adding a fix (or at least warning) to the /dev/build scripts to solve this issue?