Can someone plz correct meFollow

You will not lose any skills pernamently. All secondary job means that your subjob cannot exceed half of your primary job. So, when you first get your rng as main and rdm as sub, you will be lvl 1 rng and lvl 1 rdm. As you lvl your rng main, your subjob will automatically keep up. So when you lvl your rng to 60, you will be lvl 60 rng and lvl 30 rdm. You can switch back to rdm at anytime in your Mog House to be back to lvl 30 rdm.

Just to warn you tho, rng is easily the most expensive job and money don't grow on trees.

Once you hit level 30, you will be ABLE to do the Ranger Flag Quest. After the quest is done, then you go to your Mog House and change your Main Job to Ranger. You'll then be a level 1 Ranger. Now, if you put your subjob as Red Mage, you'd be a RNG01/RDM01. This doesn't mean, however, that your Red Mage PERMANENTLY loses these levels, but as you level up your Ranger, your Red Mage will level up alongside you. (because you've already gotten to level 30 with the Red Mage) The subjob cannot exceed half of the main job's level. So you can be a RNG15/RDM07. As soon as you hit 16, you'd become a RNG16/RDM08. Your subjob's levels would only keep up if you've already leveled that character. For instance, you couldn't be a RNG/WAR and have the Warrior level up alongside you (if you haven't ever leveled the Warrior) you'd be something like RNG12/WAR01.

A lot of people take the path you're taking. Level up what you want to be your MAIN secondary, and once you get to level 30, make that the subjob of your Advanced Job, so they'll keep up in levels. Just remember - for every 2 levels your main gets, your subjob gets 1 level. (assuming you've already leveled up your subjob.)

Hope that cleared a few questions.