I'm not sure what you mean by "a pool for solo miners", I'f you're using a pool then you're not solo mining. Typically for solo mining you just setup the wallet to run as a server and your miners connect to the server and mine on the chain that way.

If your miners are the only ones connected to the pool its the same as if they were each just solo mining by themselves.

The "diff" your referring to between the miners and pool isn't the difficulty of the coin being mined, its basically the amount of work that the pool gives to the miner to solve each time. Think of it as more of a "packet size" A higher diff between pool and miners just represents more shares being solved in each "packet"

That being said it depends on your cards speed.

For example:

500kHs = diff of 128 or 256

900kHs = diff of 256 or 512

Those are some numbers I have found to work well.