This is too powerful. Particularly the Extra Familiar ability. At 10th level, you could have two imps running around, activating magic items and buffed with all of your buffs (Nightstalker's Transformation? Mental Pinnacle? Shapechange? Even just Alter Self can be brutal). Both imps have full class levels in classes of your choice - make them both swordsages, and they're both running around murdering people, and locking down whoever they can't kill.

Both imps are half-fey, getting such gems of spell-like abilities as Dominate Person and Lesser Geas. If you're feeling nasty, send your imps to sneak - invisibly - into peoples' houses as they sleep, and use their Geas/Quest spell-like on them. Both imps are also half-fiend, but not necessarily the same type of half-fiend. Maybe one is half-succubus, giving them even more enchantment SLAs and increasing all their save DCs. The other could be half-goristro, growing in size, and getting huge boosts to natural armor, Constitution, and especially Strength. Whatever floats your boat.

Also, you could fuse with any one of them at will, getting damage reduction 5/good or silver, darkvision 60 ft., DR 5/magic and iron, fast healing 2, immunity to enchantments and poison, resistance to acid/cold/electricity/fire 10, SR 25, all of their spell-like abilities, alternate form, +2 Strength, +11 Dexterity, fly 50 (perfect), six feats of your choice, and all the class features of one class of your choice. If you fuse with the half-goristro one, your abillity increases are instead +12 Strength, +7 Dexterity, and +8 Constitution.

You get all of this, and you're a full caster, with 9th level spells?

Extra Familiar, Familiar Class, and Familiar Fusion are all way too good. The first two probably just need to be ditched; Familiar Fusion could work, but it needs to be toned down.

Infused Familiar is probably too good, but it's not completely over the top. I'd be hesitant to let it stay with Familiar Fusion around, though. Also, how does it interact with an imp, or a pseudodragon? Do they suddenly switch to Magical Beasts?

Combined Familiar just seems like it's too powerful to me; but, imps pretty much already get everything you could want to give them, so it's probably fine.

Strange familiar can be powerful combined with Infused Familiar. Half-Celestial/Fiend, Phrenic, whatever other templates that grant spell-likes based on HD.

For evolve familiar: are they limited by the level requirements?

Sorry I kind of tore into your class. Believe me when I say I wouldn't have taken the time to do so if I didn't like it.