back when I was doing a lot (15-20 varieties) of killies, I had walter/micro/bannana worm cultures going, along with vinegar eels, bbs, you name it..
in my opinion, the difference between most of the nematodes, is very slight, maybe microns, but not enough to have each one for specific sized fry.
I might suggest a vinegar eel culture for something a bit smaller than bbs, that is extremely easy to make/keep.....
50% apple cider vinegar
50% water
put a slice of apple in the bottle (i use 2 litre soda bottles) put a piece of filter floss in the top and set it on the shelf....
after a time, a mulm will develop on the bottom of bottle....no worries.
if you take a small light and sine it through the bottle from side to side, near the top, you will see tiny worms along the miniscus.
easy to harvest too....
find a long necked bottle, siphon some of the water from the main bottle to the smaller long necked bottle, trying to siphon along the top edge (miniscus) of the colony bottle. Fill your harvest bottle to withing 2-3" from the top and then stuff in some filter floss so that it touches the vinegar water and leaves about 1"+ opening to the top. Add some fresh water to the top of the bottle over the floss...
in a few hours, the little worms will migrate through the floss to the fresh water up top and you can simply suck them up with a pipette and add them to your fry tank. The worms will live a long time in the tank water and they tend to stay in the water column so easy for fry to find.
You can add some fresh water to the top of the harvest container if you take out too much sucking up the worms..when they are gone, you can add the water back to the main container.
Once You know you have worms in your main bottle....start another one to make sure you have a secondary source.... I have kept vinegar eels for many many months, and there is really nothing that will make them go bad like the micros.... just add a 50/50 solution to fill your cultures and need be and a small piece of apple from time to time...
If I can remember, I will bring a culture or two to the next meeting for the mini auctions...
good luck
david