cIOS rev20b base of 56 will work with most games, but a few games will require installing a base of 57. Thats a downfall of Neogamma, it will only use one cIOS. I saw a few months ago on the Neogamma thread that Wiipower was thinking of introducing loading from slot 248 or 249 but never saw him post anything about it since, and he has also not made a new revision yet, NeoGamma is stuck at r9 b47, maybe the next release will allow for loading from 248 and 249.
Do these games regulary use IOS57? If yes, i will consider adding an option to load IOS248 for games that request IOS57. So there's nothing to configure in that case.