Actually, both Spell immunity and SR don't affect your own spells.
This is why Golems can cast Haste on themselves.
Page 298 DMG Under Spell Resistance: 5th paragraph:
A Creature's spell resistance never interferes with their own spells, item, or abilities.
While that is true for Spell Resistance, the case is somewhat complex for Spell Immunity of Rakshasa.
First of all, there is no generic definition or entry which covers Spell Immunity of all the creatures in 3.0e.
If you use 3.0e MM Rakshasa, it says,
Rakshasas ignore the effects of spells and spell-like abilities of 8th-level or less, just as if the spellcaster had failed to overcome spell resistance.
So yes, 3.0e MM Rakshasa is immune only to spells which allow spell resistance and it can cast spells and spell-like abilities on itself without problem.
On the other hand, in SS (somewhat like a 3.25e supplement) , Rakshasa's Spell Resistance ability does not has that sentence and simply says they are immune to all the effects of spells of certain levels. So if we take it as written, SS Rakshasa is immune to all the spells of certain level or levels, not only to spells which allow SR.
And that entry also says,
Like spell resistance, a rakshasa may suppress this ability in order to accept its own spells or spells from allies.
Of course, as OP is playing 3.5e, he should better use 3.5e MM Rakshasa. Ans if he does need to play it from lower levels, Runestar's suggestion seems to be reasonable.
