Hi Otto, I returned from precisely that area about 10 days ago. Put in at Hoel Pond, traversed Turtle and Slang Ponds into Long Pond, and finally carried to Mountain Pond- all in one afternoon- after having driven 8 hrs. from southeast PA! If there are no significant headwinds you'll have no trouble with your itinerary. Site #1 in the northwest corner of Slang Pond is pretty nice, with a great fire pit and view of the lake on a rocky promontory and logs for sitting and gazing at the sunset and your campfire.
The carry to Long Pond is relatively brief, and virtually level. As Waterspyder suggested, Pink and Little Pink are worth the effort to explore... intimate little ponds with wildlife and flora to admire.
On my return, I paddled to Hoel Pond after climbing Long Pond Mountain and exploring Pink Pond, and camped on a high rocky cliff above Hoel Pond on the northern shore. If your party is big, you may have difficulty in finding a truly level tent pad however. It's worth a look to admire the view from the cliff.
I then returned to my truck, moved to Little Clear Pond and paddled into Little Long Pond (east) via Green Pond, then returning via a short carry to St.Regis Pond for my third night. I had a truly biblical deluge of rain that night, but I knew it was coming and chose my tent pad carefully for runoff drainage... and, erected my tarp over my tent for additional protection. I stayed dry but the downpour was deafening on the tarp. I wish I had brought foam ear plugs.
Anyway, I guarantee you'll have fun. You might even see me out there in a couple of weeks in the wood/canvas canoe I'm restoring. I'm two coats of paint & a few details away from her maiden voyage!
See also my June 15th St. Regis Canoe Area trip report