Can I do this to my modcon?

A buffer would be a band-aid (and an expensive one, if going with a BoilerBuddy or ErgoMax). Right-sizing the boiler is a much better approach, but as pointed out previously, even with the -050 it'll still short-cycle on the fin-tube zones unless you marry them into a single zone or increase the radiation on each of the small zones.

At 180F out it's probably getting ~87% combustion efficiency. If it's running 180F out at 37% of the -100's max fire that's (0.37 x 0.87 x 100K-in= ) ~32KBTU/hr, and you're already at the highest temperature that the PEX is rated for. If your radiation can't take anywhere near the full output of the boiler without melting, it's dead-certain that it's the wrong boiler, eh? ;-)

ToddinMaine said:

Our radiant is clip-finned pex suspended under the floor - it heats the airspace beneath the floor so the design temp is 180 deg, a good match for fin-tube. It's a rather common set-up in this area.
Plus, we have alot of large double-hungs upstairs, so the space from the window sill-to-floor is limited making BB's a good fit.

Click to expand...

The finned aluminum butterflies on PEX approach to radiant makes some sense if you have an oil-fired cast iron boiler that can't tolerate condensing temps, but is wasting the potential condensing efficiency of a mod con.

Convecting panel radiators come in baseboard-heights if necessary and deliver more consistent output at low water temps than fin-tube. Ten feet of 12" tall panel radiator delivers ~9-9.5KBTU/hr @ 180F AWT, and is still good for ~5KBTU/hr @ 140F AWT. A 10' section of 8-9" high panel is good for ~7500BTU/hr @ 180F AWT. It's more money than fin tube (even the cheaper ones), but it's also more comfort, and capable of reaping lower temp/higher efficiency operation out of the boiler.