As far as we're concerned in regards to support classes is for the promoter to run as many as they see fit.
Institute a reasonable time limit on the support class heats and B-mains and a two spin rule.
Also, be sure to run one of the support class B-mains following the headliner B-main to give the guys who transferred from headliner B-main a chance to work on their racing machines. Then run the headliner A-main
first.
No dreaded downtime, everyone who wants more racing for their money gets it and the headliner B-main guys have a chance to tweak their cars prior to the headliner A-main. Those who don't care to watch the support class features can happily be on their way home.