Meegs — Sowerby pattern numbers were always strictly sequential. So a plinth could have the same number as the bowl or vase it was made for, or a slightly later number, or a much later number, as in the case of the 2505 Posy — a later size. I haven't found any examples the other way around, but it's possible. Once a general purpose round plinth had been made it became available for anything appropriate it would fit.
My experience of 2525 frogs and bulrushes is that it turns up most frequently without a plinth, then on a general purpose round Sowerby plinth. Obviously the 2528 and 2531 specials were available at launch, but how they were sold is not known. For example it could have been 2528 for bowls and 2531 for vases. Some wholesalers and retailers might have preferred a standard round plinth for simplicity of stock control and ease of replacement in the event of a breakage.
The Sowerby 2487 star flower holder is the only correct fitment for the upright 2525 vase.