a quarterwave at 18 MHz would work as a 3/4 wave on 54 MHz, so yep, that should work. As for the balun you could make one out of coax that should work for both bands. I would try the 1.8-30 one you have at real low power, just to see what the SWR is...who knows, it might work. And if its only for receive, just leave the balun off. I never use a balun for my dipoles, and transmit up to 1200W with no issues, equipment wise.
Scanners: BCD536HP BCD436HP BCT15
Ham: HTX-212 Yaesu 757 Kenwood B2000