The 2E RPG seems clear that the linkup is for the Alpha/Beta only (as that is all that is addressed), and Destroid pilots are discouraged from space operations off of the hull. Which is true, but the question is SHOULD (specific) UEEF Destroids also be given the ability to connect to the Beta fighter as an Alpha-Battloid stand-in? For the purposes of this thread I am restricting myself to the 2E RPG universe (I have done the number crunching with 1E and will post numbers if anyone is interested, for the most part what is basically true for 2E applies to 1E though there are exceptions) and not the series (or novel or comic). I am also limiting my initial look at just the UEEF Phalaanx Mk.XV and the Defender Mk.XIV, some units are obviously not going to work (size) or do not bring anything substantive to the table.
Pro
Doing so comes with some advantages shared by both in terms of production cost/efficiency as a non-transformable battloid is going to be easier and more efficient to produce.
Making them mobile gives the Destroid a better firing arc/positioning than if it was on a ship's hull.
Both Destroids would be force multipliers in certain roles, essentially doing the job of multiple Veritechs for certain missions:
-The UEEF Phalanx
-The UEEF Defender
In terms of fleet air-defense in space, a UEEF Defender can successfully engage and take out more of the same target than an Alpha with its SRMs and do so at a longer range. Longer reaching missiles (Medium/Long) would technically outclass the Defender in terms of engagement zone, but these types of missiles are typically carried in small numbers (compared to the Alpha's SRM capacity) forcing them into range of the enemy weapons where a Defender could potentially hold them to outside this zone. A 24-element Squadron of Defenders could theoretically repel a Calm-ship's worth of Invid mecha (w/o boosters) before they could enter firing range, where the Alpha equivalent would be forced to close to gun range against similar numbers (assuming missile volley of 4, if they can do it with 2 missiles it would be doable but they do not have as much "staying power" in this region before being forced into gunrange).
Con
Nothing, AFAIK, indicates the Destroid/Battloids (in general) can survive re-entry operation. Which means it would require additional add-on to the Destroid or return to their mother ship.
It might require a pilot in the Beta and the Destroid, though it could all be done with the Destroid I would think (using the Alpha as a baseline) but the Destroid pilot would need additional training.
The Beta will suffer some flight penalties due to Destroid mass.
Timeline details of the Beta also limit this to a late 2030s or later period. Off hand I am not sure what the service life looks like for UEEF destroids in general, much less specific types.
Mix
Dimension wise the two destroids looked at are actually shorter than the Alpha Battloid, but both are also wider, but in terms of length it goes both ways. Depending on how these dimensions overlay on an Alpha Battloid in the docking position, it could make a tight or impossible fit.
The Docking Hardware's ability to support and secure the mass of the Destroids: the Defender is lighter than an Alpha (so no issue here), but the Phalanx is several tons heavier in a dry state (which brings into question if the docking beam could support the added mass especially once we get into the fully loaded state).
Conclusion
Based on the above having the Beta docking ability would be a great asset to UEEF fighter operations in space and potentially explain deficiencies in the UEEF Fighters (light MRM/LRM capacity) and act as a force multiplier for certain mission sets.
However specific aspects related to their size (dimension and mass) and the timeline would I think force a negative conclusion for the use of the Beta Fighter specifically, but would not rule out the creation of a dedicated option to give them "space mobility" on par with the A/B combo (and depending on how its designed and its performance might also work for the Alpha).