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Your pipe fitter is probably correct and is looking out for your best interests, IMHO.
Here is the cautionary fallacy about reverse-return piping:
It allows the same "potential" pressure differential at each unit served. It works (by "works" meaning intent of being self-balancing) only when the unit's served have the same pressure drop. It is doubtful that your WSHP's have close to the same PD's.
The other is that reverse-return piping as a design concept works the first time you design it. Unless provisions have been made in the form of extended header pipe sizes with tees and valves, any addition to the system is going to "violate" the design precept.
(If the system was important enough to design reverse-return in the first place one has to assume that the concept is essential to it's operation.)
Enough theory; back to reality: Will one heat pump at about 1/60th of the total throw things out of whack to the point where the others will be adversely affected? Probably not if it is remote from the pumps; Probably so if it is close. But how do you know that this is the first one added? Takes a big picture.
Here is what I do for WSHP jobs: I use Griswold constant flow valves. WSHP's are rather fussy about constant water flow and this is the only way I know of to approach this. Your other WSHP's may already have them; that alone is reason to do so in this case.
My first course of action is to exhaust the option of "doing it right", tying to the tail of the return main even if you have to chase it to a larger pipe size. And if Griswold's are installed elsewhere, here you must also.
Here is my second choice of action: I would at least for the one you are installing, put in a Griswold valve. If you do not, or if manually balanced and someone tampers with it, you create a DP bypass where you need it least, close to the pumps. That one path could thwart flow to the others, it is true, and be noisy in the process as it fights total head.