1. Run a lvl 7 or 8 adventure.
The paladin will have a rough time, and the ninja will probably be bored. Make sure the challenges aren't easily circumvented by the ninja's abilities, but actually require the other characters to take an active part.
2. Use the XP table in the DMG.
this will reduce the XP progression of the Ninja to a crawl, while making sure the paladin will get up to speed.
3. Don't give out story awards unless you know exactly what you are doing:
DMG p.40 said:
Sometimes you may want to estimate experience point awards for actions that normally don’t result in an XP award under the standard system. These are called story awards, and they should only be used by an experienced DM.
4.If you DO want to keep giving out XP for good roleplay, follow the advise in the DMG:
DMG p.41 said:
XP awards for roleplaying are purely ad hoc. That is, no system exists for assigning Challenge Ratings to bits of roleplaying. The
awards should be just large enough for the player to notice them, probably no more than 50 XP per character level per adventure.
Note that suggestion 1 will never work if you ignore 2-4.
Personally, I don't give RP XP to my group, for the following reasons:
1. They all roleplay. Making a distinction would be hard.
2. I'm biased. I know beforehand I will notice some good roleplay, while missing some other piece of brilliant interaction altogether.
3. I have a hard time keeping the characters at the same level as it is
One of my groups now contains a 7th lvl barbarian, an 8th lvl wizard, a 9th lvl duskblade, a 9th lvl cleric, and a 10th lvl rogue.
Until (very) recently, the barbarian was lvl 5, the wizard lvl 7 and the duskblade and cleric lvl 8. The rogue has beenlvl 10 for quite some time.
Of course, that was also when I was still using the OD&D rules for those characters..... Only when i switched to 3.5 and used the DMG XP tables did I get anywhere near progression.
Now, if you have the same problem as I did (not enough XP per session to see any progression based on the amount of 'kills' they make, and not enough sessions per year. I'm happy if I get to play 3 times a year with this particular group....) just double, triple or quadruple the amount of XP per challenge. The progression will increase, without losing the inherent re-balance system presented in the XP tables in the DMG.