>I have an EXE that someone else created using FPW 2.6 which uses FOXW2600.ESL. I would like to create a new program in FPW 2.5B which would call this executable. Would I need both the FOXW2600.ESL and the FOXW250B.ESL or would just one be fine? If just one, does it matter which one?

You have to have ESL file corresponded to the version of FPW you are using. In case of FPW 2.5B you'll need FOXW250B.ESL only.