primary loop circulator

Need to fix a job someone else did in a church. 2 Weil-McLain ultra 230 boilers. Installer did a pretty good job of primary/secondary piping, with 4 secondaries, each with their own circs off a 2 1/2" primary loop. Using one circulator on each boiler (supply side, very good so far) as injection circs. Seems the only thing he forgot was a primary loop circulator. The loop is not especially large, doesn't need a huge circ. The largest flanges I am aware of for a cartridge style circ (Taco 0011 probably) is 1 1/2". Can I reduce the 2 1/2" pipe to 1 1/2" just for the circ without suffering too much? Or are larger flanges available for these?