Hi T.,
I applaud your courage taking on 5 Y chromosomes in one family! I do not envy your food budget in the upcoming years. As it is, my 14-year old son gets excited when I allow purchase of THREE gallons of milk on a single grocery trip. And I am only outnumbered 3:1!
I wrote a perl script some time ago that reads in "the complete chore list" and spews out individualized assignments. Chores can be weighted, and different "total chore loads" can be assigned to different members of the family. I'll take a poke at it to see how easy it would be to incorporate other constraints, such as day-of-the-week availability. If you're on a Macintosh, I can send it to you and it will run automatically. If you're not, I can help you set it up to run, but it might be more work than you wish.
In any case, I think the ALGORITHM and RULES are reasonable.
1) Anyone can trade anything to anyone else as long as both parties agree (documentation/communication left to personal choice). [BTW, negotiation skills are something WELL worth teaching... feel free to ask me more on this topic]
2) Chores need to be adjusted based on ability. In your case, "higher difficulty chores" can be assigned to "his" kids, but the "total points" should be reduced on the assumption that they're also carrying a chore workload at their mother's house.
3) Randomness... list changes week to week.
4) Kids can step up and ask for more. I have a hard time thinking that "half time kids" MUST perform "full time chores". In your situation, I would ASK HIS kids what they want. HALF expectations both ways or to double up. I'm assuming that they want the double-up option, based on your question.
I would make sure that DAILY chores are assigned to his kids whenever they are in residence. And, make them all aware that that you acknowledge this "favoritism".... put it out there, with a reminder of "partial residence". When the part-time interlopers fulfill full-time chores, I suspect all will bond.
--S.