^ This.
@Daddylouc: think of your basslet as a classical guitar. There's no law against replacing its strings with steel-core strings, but it would immediately suffer from intonation problems (the bridge was originally placed with flexible strings in mind, and has too little compensation for steel-cores -you'd have to move it back), and possibly exhibit medium- or long-term structural problems (the top and its bracing are likely to have been designed in order for the bass to provide *some* acoustic volume, and may be too lightly built for all-metal strings).
Have a look at this thread for alternatives to the D'Addario EXPPBB190GS set:
Guitar Scale Acoustic Basses
Cliffs Notes: there are some options, but nothing flatwound or tapewound that fits.
[There is one set of each type, both made by Galli Strings (KA-BASS-4FW, made for Kala, and UXB810, respectively) that would work, in principle, because they have synthetic cores: unfortunately, their winding length is
probably not enough for the Guild (they're made for ukulele basses, which have shorter scale lengths).]