Both the fixture and the bulbs are E26 standard medium socket; so they definitely fit.
And I'm pretty sure the wattage is fine (42w < 50w).
I'm just worried about causing a safety hazard... Would combining these non-"R20 Halogen bulbs" into this "R20 Halogen bulb only" fixture create an unsafe lighting installation?
I should mention that the fit is rather tight, and that although 100% of the bulb and 50% of the base of the bulb are uncovered, the bottom half of the bulb's base is tight against the edge of the fixture.
Please let me know if additional information is needed.