Having once done an essay on the wider aspects of Roman military familia (families as well as slaves) I would second Klingan on the extreme difficulty you may find in obtaining enough relevant material to build up a decent Masters thesis.
From memory there are a few reference books around where you may find some relevant information including:
Campbell B. (1978) The Marriage of Soldiers under the Empire, JRS Vol 68
Campbell B (1996) The Roman Army 31BC - AD337 A Sourcebook, London Routledge
Evans J.K. (1991) War, Women and Children in Ancient Rome, London Routledge
Goldsworthy A. and Haynes I. eds. (1999) The Roman Army as a Community, Journal of Roman Archaeology Supplementary Series Number 34, Portsmouth, Rhode Island
Saller R.P. and Shaw B (1984) Tombstones and Roman Family Relations in the Principate: Civilians, Soldiers & Slaves, JRS, Vol. 74
NB I would not recommend Frier B.W. & McGinn T A J A Casebook on Roman Family Law since there is no real index and only a few apparent mentions of slave women without any obvious link to the army.
There are also a small number of laws such as those listed in the 'Digest' (part of Corpus Juris Civilis) which relate to soldiers and slaves particularly regarding inheritance and soldiers wills.