Well I can't give you any good advice on mixing the fry . I've done a ton of research on rearing erectus fry, but don't quite remember any info on exactly that (it's been a year or so). I would search the forum at seahorse.org, you should certainly find info on it. Or post the question there.
If you can tell your male is about to burst, imho, I would take him out. You could probably wait until the morning though to reduce stress. Seahorses almost always (in my experience and what I've read) give birth at first light. Mine have always done this. They start twitching and acting funny like they have parasites soon after the lights turn on. I used that as my key to get him out and they always seem to burst with in minutes of being pulled out, then I put the male back. It saves so many fry from getting sucked into the filter, caught by mouths in your tank and lost in rock. Also, I feel like it reduces any damage to the fry from sucking them out with airline tubing or a turkey baster. What time did your male give birth last, just curious?
More babies...WOW