The EMD F3 B-unit and EMD F7 B-unit has the wrong mass (negative? lol...maybe only in description)

Could we add 2 B-Units?

I assume that B-units will be supported with the rewrite?

The rewrite will have more proper B-units, but for now the current ones in 013 work pretty well. More could be added for 014 if someone wants to make them.

But will the Physics system support them without having to use weird hacks like this to make it work well?

yeah, the physics in the rewrite will allow it to properly effect the train's functionality as if it's another train.
it would still get managed like a tanker (for ease of use), but it wont be so hacky as to need the weight to be negative.

You should have googled first what the b-unit does.
negative mass is correct and the type is correct too.

A B-Unit is the same as an A-Unit (EMD F7) just without the Cab. It's a second loco to make it easier to pull heavy load. Since our system doesn't support B-Units like they act in reallife we reduce the mass of the whole train to emulate this.

But you need to fill it up with diesel otherwise it's not working.