Family visa

its 3 or 6 months, this period depends upon your sponsor/ comapny. and i guess once you arrive at oman, the first thing you need to do is to get resident card, and after that you can leave country any no. of times, till then, as the visa is the only permit you have with which you have entered the country, can be considered as single entry. but that is not a problem since after arrival you can get the resident car within 15 days, and you family can get a family visit visa (after talking to you sponsor/pro). After the period of 3/6 months time you can apply for family visa.

I would say , you can apply for family visa as soon as you get your resident card and your employers are okay with it .

Technically , you can get your resident card on the day of your arrival and you can apply for family visa on day 2 itself . I am assuming the medical report from GAMCA for your spouse and other documents are ready .