You have it right, just leave the 12B4A pin 3 and the 12AX7 pin 9 unconnected. The filaments will be running from 12.6VDC, so this will work out well.
Even though 12B4As and 1626s are different, they have a "coincidental" compatibility in that they will be "happy" with the same B+ voltage (appx 240-245 VDC, and plate / cathode currents (25-27 mA) and cathode resistor (1K, 3 to 5W), and SE OPT primary impedance (5K to 7K).
The 12B4As will deliver about twice the output power (1.6W) as the 1626 (750 mW). Interestingly, in the 1626 data sheet, it states that these tubes are not suitable for purposes other than RF oscillators or Class C RF amplifiers. I guess they forgot to tell the tubes about that, they sound quite nice in a low powered SET like the "Darling".
I suppose that one could build up an amp with both octal and 9 pin sockets properly interconnected, and have the choice of operating it with either type of tube.
I've seen a similar technique used before, in one of teh Angela Instruments DIY projects, it was wired up wit both types of sockets, so that either a 12AX7 (or 5751), or a 6SL7 could be used as the VA stage.
If you prefer using octal tubes for the VA stage, just use a 12SL7 in place of the 12AX7, they are close enough that no changes (other than the tube socket) is needed.
/ed B