Juli – None of the worms or pieces will float. I don’t recall if the rice paddy frogs fully submerge or not but I would think they would dive to take a worm. For a really great set up you should have some sloping shorelines and perhaps a rock tower that just clears the surface. The Rice Paddy frogs will just lay sprawled out with their nose and front feet on the edge of whatever they can rest against. I had some for a while a few years back and just loved them. 
So, what I would recommend is that if you can set up a sloped shoreline you feed them the worms in that area. I would think they would get used to checking there for food once it becomes consistent. The water would be shallow so they could easily clean up the worms in the shallow water there. I would probably still feed a few crickets but this should eliminate the need for dusting or at least could let you cut back to once a week or biweekly.
My recommendation would be either red wigglers or European Night crawlers. Both are small and not too tough to get started. You can put euros and reds in the same bin. I have a large sterlite tub filled with ground peat about 10-12” deep. Make the soil moist but not soggy. If you can squeeze water out it is too moist. If you remove a worm and dirt is stuck all over the worm the substrate is too dry.
To feed the worms make a trench down the center of the tub and put in a nice layer of chopped waste foods. You can use all vegetables and fruits, tea bags & leaves, coffee grounds, and egg shells. Don’t use meats, dairy, or fats. You can also add shredded paper if you are in short supply of garbage. What you will be doing is setting up an indoor compost pile. The reason for chopping the food is so it is consumed more easily. In a garden compost pile you would just alternate layers of soil, garbage and garden lime. In the house you want the process to run a bit more quickly so everything is on a smaller scale including the food pieces. I never throw away banana peels, citrus rinds, apple or pear cores, peach pits, and corncobs, husks, and silk. All go into the tub. If there are enough worms and you have a lot of food you can trench down each side and use two trenches. I alternate from the center to the sides. If you have too much food just chop it, place it in a zip lock bag and freeze it until needed. If smell becomes an issue the soil will need to be sweetened by sprinkling a little garden lime over the surface and lightly watering it in. The lime is also utilized by the worms for breaking down the food so that it can be digested so a sprinkle of lime from time to time is a good idea anyway.
When table scraps are in short supply you can feed chicken layana or commercial worm food. These must be top feed rather than buried to prevent toxicity in the soil.
For more information on types of worms and methods of composting indoors check these two sites:
Happy D Ranch – http://www.happydranch.com
http://www.happydranch.com/articles.html
http://www.happydranch.com/worms.html
http://www.happydranch.com/wormtalk/index.cgi
KAZARIE Worm Farm - http://www.kazarie.com/
Must read topic here is ‘Make a worm bin’
As for the snails, I have read that the MHDs will also relish snails but am not sure where to get them or what type would be advisable. If you find out anything on snails please share it with me!
If you have any other worm questions let me know and I will try to help.
Oh, in case you haven’t yet found it there was an article in Reptiles or Vivarium magazine a few years back. I don’t recall which it was but am sure I still have my copy (I never throw these away) and can look it up for you. I did keep a few for a while but when I lost them I went with terrestrial frogs and then eventually turned my full attention to my MHDs. I do have a few snakes but will be cutting back there too.
I know what you mean about keeping the creepy crawlies, anything for your pets! 
I never though I would keep roaches but just bought a starter colony of my second type of roaches, now I have 2 kinds of roaches, bins of mealies, wigglers, and all of the rodents.
Life is good!
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Marcia - FroggieB Dragons
www.froggieb.com/MHDHome.html