The problem with that I think is that the game would view it as:
1 Ranger
...
6 Ranger
7 Pally
8 Pally
9 -MONK MERG ERROR 4 Classes freakout.
because it thinks the rest of your character is:
10 Rogue
...
20 Rogue
So you couldn't take that first Monk level. But, as Unacceptable posted, if you're happy to use a +3 Lesser Heart instead of a +1, you could do this with your LR +20:
1 Ranger
...
8 Ranger
9 Monk
...
15 Monk
16 Ranger
...
20 Ranger
Then use the LR +3 Heart to change the 8 Ranger / 12 Monk build, and swap your level 7 Ranger class to Paladin and your level 8 Ranger class to Paladin.
So you end up with the split and levels you want.