You can change job whenever you want at your mog house and you can take it back later with the same lvl as it was. So if you are a lvl 70 RM you can change for WM or any other job. If later you can take summoner back and it will still be at lvl 70. So it is possible to max your lvl in every job.

It is possible to have multiple caracters per server(I think it's 8) but it costs 1$ per caracter per month