I'd like compatibility with Railcraft via making modern industrialization able to use railcraft steam since they're both fluids by forge standards. You could also make it an config option so people don't skip the mi boilers

It's a bit complicated. Boilers are locked to only accept steam, and this locking system only accepts a single fluid. It is difficult to accept multiple versions of steam.

In this case, can you make the steam inputs to not accept the fluid at all? Because the current situation is the machines are do accepting Railcraft steam as fluid just does not work with it, which causes a confusing situation.

Alternative suggestion is a - cheap - fluid adapter block or cover, that can accept various fluids categorized as steam and converts them directly to MI steam. It could mean BigReactor's steam, Mek's steam, whatever, AFAIK there is a whole tagging system for liquids that can be detected and converted to steam.

Blocking certain fluids from being used in machines would be even more confusing. MI likely will not add new blocks to convert steam fluids. That would be better fitting for some standalone mod.