no, it won't conflict.
at worse, it will replace what you already have (overwriting it).
at best, it will update it.
just note that common setup in most guides tell users to install 2 different cIOS :
slot 249 : a cIOS based on IOS 56 (lot of games are working, but not all)
slot 250 : a cIOS based on IOS 57 (more games are working, but it's slightly slower than the other one).
By default, loaders are using slot249, so most guide tell users to switch to 250 in the loader's setting.
I don't see the point installing a bad cIOS in the default slot just to not use it anyway.
that's why you found my post saying to install the base57 in slot249 (default slot = best compatibility)
So, if you followed an old guide, maybe you already have the dual setup I mentionned.
installing base57 in slot249 might just ends having both 249 and 250 with the same cIOS base/version, replacing your 249(56) with a 249(57).
you'll have :
249 base57
250 base57
there's no point doing that. Just edit your loader's setting to use slot250 if you already have base57 installed.