The Lato font collection includes hairline, light, medium, semi-bold, bold and heavy weights. Presumably these are designed with more visual sensitivity than a stroke width generator would produce. So enjoy your 18 variants of Lato font.
The only way I see to do this within FCP7 is to install just the one variant you wish. Then what FCP7 calls Lato "plain" is exactly that Lato variant you've installed.
FCP7 can't handle families of font variants. FCP7 grabs one member of the family to be the "plain", and from this FCP7 cobbles a bold, an italic, and a bold-italic variant. They are not the Lato variants.
The 18 Lato variants come as separate .ttf files, but the family name "Lato" is coded somewhere in each binary file so they always install in that family.
It would be nice if you could merely disable the 17 variants you don't want for a particular title using that Font Book function. But FCP7 ignores Font Book disabling.
* (At least it does under OSX 10.8.5 where I tried.) You must really uninstall the 17 to get the one you want.
Dennis Couzin
Berlin, Germany
*[note added 25 November 2016: Refuted in later posts.]