Re: High lakes above Mammoth
I have done the river trail hike to thousand island lake out of agnew meadows about 5 or 6 times. You'll love it. Don't overlook garnet lake, it's the best of the bunch on that hike, with excellent fishing. The outlet of the lake has lots of fish.
The hike from the San Joaqin up to garnet is a tough one and not well marked, though you may be taking another route, and the connecting streams from ruby lake to garnet are mere trickles, so for stream/river fishing the outlets of the lakes that are going directly to the san joaqin are your best bets.
I've mostly spin casted up there but can assure you the fishing is excellent and they don't seem particular by any means. Let me know if you have any questions as I've gotten to know that area quite well.