Nice train!
Yes, some more coaches would look fine.
But prototypically there often was a combination with 3y´s and one or two 4y´s. THAT looks fine, too.
Or even better: the 3y-pair, a 4axler and a 4 axler wooden express baggage car. Oh there were lots of nice combinations!!!
No worry about the composition. The AB3y/BD3y was common. Nearly all thinkable combinations were put together.
All based on 3 types of cars:
the B3y (all-second class, 3 axle, rebuilt-program)
the AB3y (1st/second class combi)
and the BD3y (combine with a baggage-part)
As far as i know there never was a AD-Combine, just for the case that will ever come to discussion (or a chinese cheap-builder will make one).
So, your AB/BD-combi is very fine and fitting the 218 perfect, even though the 2500PS may be a bit overmade for this short train. As i told you: ONE 4 axler or a second pair (but then a B/B-pair) and it will look perfect!
Those Coaches were built on standardized (while rebuilding) frames of old 3axle Reichsbahn-Cars.
They all got the same hull and interior and were well known for their bad running in higher speeds. Coupling two cars tight together gave the cars a power-guidance and the speed could have been risen by 10 km/h.
In the same shape there were the 4-axle rebuilts, looking exactly the same with same interior and the same classes:
AB, B, and BD, but with additional middle doors and each running on its own. But especially on the 4axlers u can see their splitted home: there are 4 or 5 different trucks known, and in detail a lot more: all coming from old Reichsbahn-Coaches.
I wish u many fun with your cars, dont mind to ask if u want to know something about them.
Even though i have to confess that the brake-lettering on the side confused me: i thought i KNEW about it...
Greetings
Frank