It is certainly correct and idiomatic to say it.

Dans un mois et quelque(s).

means, "in one month plus some (days)" if you use the plural (Dans un mois et quelques jours) or plus some unspecified amount of time if you use the singular (Dans un mois et "quelque temps").

You might also use the idiom:

Dans un mois et des poussières.

to state a very small amount of time need to be added to that duration, and for a larger amount, that might be:

Dans un mois et des brouettes.

For in one and a half month, you would say:

Dans un mois et demi. (one month and a half)