...Using lower rated bulbs will just mean they are likely to fail faster, but that may mean hundreds of hours and (depending on how often you drive at night) may not be a big deal. 2 steps you may consider instead, first is to bypass the rheostat---the next is to replace with LED bulbs. The LEDs will at least allow more brightness without baking up the heat around the bulb (something that is certainly happening with the 6v bulb).
I haven't tried this, maybe there are others but at least the engineering here is smart; https://www.pantera-electronics.com/ledgaugelight.htm . And the price is really not so outrageous ($40 for 6 bulbs and 4 times brighter than the incandescent type...Also, has a hybrid green/white, so less likely to have the bluish tint inherent in most white LEDs). See also from our other friends at PIM https://www.ebay.com/itm/DETOM...wridaFHfw:rk:22:pf:0 . Lastly, #194 replacement LED bulbs are cheap from Hong Kong (search "t10 5050").
Plug and done.
LEDs cannot use the Rheostat well--or properly, LEDs don't work over a big range via voltage control (they have to be pulsed to have much range)...but since the problem in these old Italian gauges is never that they are too bright 
...Lee