We worm lambs only when we see a need for it, and we use albendazole which has a 5 day withdrawal, so doesn't interfere with sending them off.
However, the last two years we have needed to fluke also, so then yes you want to be giving them a dose of a combined wormer+flukicide, or just flukicide early enough that they are clear of withdrawal in time for sending them away.
No reason you can't crovect at the same time as worming. But, two things. 1. Don't get crovect on yourselves, it is extremely noxious stuff. So I'd go through drenching all of them and
then crovect them all, otherwise you will be rubbing up against crovected fleece as you drench. 2. It's pointless crovecting when it's raining
. If this is the only time you can do them, can you keep them under cover (so dry) - but well ventilated!! - for a couple of hours after spraying?