The term slim mini refers to dolls of the 1/4 (or MSD) scale that have slimmer measurements than that of the standard mini. They do not nessesarily have to look mature to be counted as a slim mini, they simply must have measurements that are similar/comparable to, for example, an Alchemic Labo Unoa or a Fairyland MiniFee.
These dolls usually look out of scale with most 1/3 sized dolls due to their smaller proportions (whereas standard minis are made to be a bit chunkier and look like children next to larger dolls), but this can be highly dependant on the stylization of the two dolls if they’ll mix in together well, proportion-wise.
Because of their smaller size, they don’t usually fit well in standard mini clothing (even tight fitting clothes can look loose/baggy, and they’ll just be swimming in pants and other bottoms). Using search terms like “slim mini”, “Unoa”, or even “MiniFee” will usually yield the best results for clothing that’ll fit your doll.
A similar doll term to slim mini is mature mini, which is sometimes used interchangeably with slim mini since they have a lot of crossover with each other.