The plural of "ufagio" is "fagio", but as Astraea368384 said, the plural of "kifagio" (a small broom) would be "vifagio".
Which syllable has stress in this word? Is fa or gi the penultimate syllable?
Gi. It's u-fa-GÍ-o.
It's not like Spanish, where two vowels together can be pronounced in one syllable - Swahili spelling uses w and y for semivowels, so each vowel (a e i o u and not y) should be a syllable of its own, so if you see a word ending in io, it's not pronounced yo.