In my personal opinion, the J-20 has small dorsal fins and ventral strakes because the reduction of the dorsal fin is ineffective.Maybe it's not that big, just long, the small, back swept vertical stabilisers and the closeness fo the two engines suggest serious attempts to conform to the area rule, along with the inherent low drag of the highly low aspect ratio delta wing, the low super-sonic trim drag of an unstable delta canard layout, the Chinese might be trying to build the least draggy of all the 5th gen fighters, probably to achieve supercruise with less powerful engines.
In terms of manoeuvrability, unless the Chinese are absolutely out of their minds, the delta canard design will be unstable in the pitch axis, and the small vertical stabilisers means instability in the yaw axis, and the ventral fins may simply be a safety measure for the initial flight testing.
Studies done using thrust vectoring shows that thrust vectoring can be used to control the air vehicle in yaw and give lateral stability, thus allowing for a reduction in vertical fin size.
the T-50 uses thrust vectoring as a lateral control device.
The Chinese were trying to reduce the fin size but without much sucess at least in this model because either it has no thrust vectoring so it needs extra fin area and the ventral fins are used to stabilize the J-20 laterally.
Now all wings highly pitched are unstable because they stall; by using LERXes or Canards they delay the burst of the wing vortices allowing for some histeresis, on a fighter like the Su-27 this allows for the Cobra.
But here is not the whole story, the forebody also creates vortices that interact with the wing, canard o LERXs, this can generate yaw unstabilities.
The forebody Chines on the F-22 does stabilize it in yaw at high AoA, and it is the same for the T-50 and J-20, their chines interact with their other vortices.
The LEVCON on the T-50 must control the aircraft better than the canards on the J-20 since it does not have any ventral fins..
Many jets need ventral fins because of insufficient dorsal fin area, in example the MiG-25, F-16 or the MiG-23,
Some aircraft have enough rudder and keel area to control the fighter with just a single fin, in example the Rafale, LCA or the Eurofighter.
Vortices and vortex burst can induce buffeting and lateral unstabilities,
The J-20 has very likely insufficient dorsal fin area, thus it needs ventral fins.
On the F-22 is clear the americans saw the F-22 needed large dorsal vertical fins, so the F-22 does not need the x fighter appearance of the J-20 with its ventral and dorsal fins.
If you have read the history of the Su-27 you will know that the integral layout with podded engines makes for the least drag, by creating smaller semi-independent elements with lower drag than a fighter with conventional configuration like the F-15 or F-14 where the intakes and forebody are blended into a single unit tha does create higher total drag.
This tell easily the T-50 is very likely the one with least drag of the fifth generation fighters.