So based on these vague cargo rules in your opinion about how many medium drones should be able to fit in a bulldog with all six seats taken?
Well since the Modification slots are based on Body for each of the various modification categories (ie if you have a body 12 vehicle you go 12 pts possible in each of six categories- with body work being one of them) I think I am going to go with Cargo Storage = unused Mod slots in the Body Modification Category. Now this houserule will have to have the caveat that some of the items of the Body Mods do not count toward space lose. The Chameleon coating is external and while it does cost Mod Slots, I would say that would not count toward cargo space loss, but these are few as most of the Body modification category are things that do take up internal space. Plus there some slots in the other Mod categories that should count toward internal cargo space.
Again this is just for regular vehicles as the bigger RV style vehicles, boats and other devices are going to have different spaces inside so a yacht will probably have loads more space for it's body than a van will.
The tricky bit is trying to figure out how much space the drones take up as using Body for them doesn't work that well, even the micro/mini drones still have Body 1 afterall.
Edit : some of my original math was off--recalculating
A general rule of thumb may figure cargo slots are roughly 0.125 cubic meter (aka a cube 0.5m x 0.5m x 0.5m ) so it may look like this:
Microdrones= 64+ per 1 slot Cargo Space (this is where it gets real fuzzy numbers as some minidrones get damn small, basically handwave it unless doing stupid amounts)
Minidrones= 8 per 1 slot Cargo Space
Small Drones =1 slot Cargo space
Medium & Human sized Anthromorphic Drones = 2-3 slots Cargo space (Trolls come in around 4-6 slots if you are stacking trolls in back

)
Large Drones = 6-8 slots Cargo space
So the Bulldog has base body of 16. So I have 16 slots for Body modifications on the vehicle and 16 slots for Weapons Mods and spent 3 slots for secret compartment just because it's where we can keep the snacks without the damn troll decker digging into them all the time. I also go with Chameleon coating because it could be handy changing the van's colours. I also put in a standard sized weapon mount that pops out when we need it (aka internal visibility) which takes up 4 slots (2 for mount, +2 for being internal visible- so when not in use you don't see it on the outside) from my Weapon Mod column.
That leaves me with 11 slots for further modifications in the Body section, 12 slots for weapon playing and 9 slots of effective space left. (16 slots for Body mods, minus 3 for the secret compartment, am not counting Chameleon toward cargo space but that internal hiding weapon mount from Weapon Mods does so minus 4).
Now lets say you have one Ares Duelist and two GM-Nissan Doberman.
That's roughly 9 Body worth of drone (assuming the upper use of 3 slots for medium, slimmer models might only be a 2 ) so you should be able to fit everything in the remaining cargo section but then you won't have much room for anything else unless its sitting on someone's lap when they are in their seats.
Some drones may also fold down some in their downtime and take up less space so if the Doberman in inactive mode folds down a bit (making it a 2) you could stack another on top (3 for the Duelist, 2 each for sleeping Dobbies so could cram 3 of them in for a total space taken up of 9)
Now suppose you went with additional seats increasing seats from 6 to 9 (50% increase) for 2 Mod slots from Body modifications. This would drop your free Cargo space down to 7 slots, but I would also say that is just when the seats are empty, because you can stack stuff on the added seats. If someone is occupying the added seats that would pretty much eat up a lot of the cargo space at 2 cargo slots per person (figure even if the person were 3 slot sized part of this is already factored into the seats), leaving just 1 cargo slot for goods,/drones. So you could seat the Duelist in one seat and 2 of the Dobbies in pack mode on the other two seats but not much else beyond that 1 spare cargo slot behind the seats.
Remember the regular seats already there do NOT count toward reducing further cargo slots nor do people sitting in those.
So if you are only carrying 5 people you could use that 6th original seat as extra cargo space using the math above.
If we had left the weapon mount as external only that would have saved us 4 slots of cargo space as external only really shouldn't count toward internal space, and then you could have loaded up some more drones or maybe some cargo like that Executive you just extracted and hog tied before throwing her in the back of the van.
Also note we have not stuffed anything in the secret compartment, but will leave that to the GM to decide what you can fit in there.
Again it's very rough and will need tweaking but could serve as a starting point.