Is the level requirement for class unlocks only for the basic class

According to KlayBuddy on GameFAQs, there is still a cap:

Statiscian, Brokers, and Managers are capped at 300%, whereas Armsmaster is capped at 1900%.

NOTE: Even with the Foresight glitch, the cap is still the cap. It does not matter if you have 2 sets of Statiscians, a Statiscian with <Insert some big number here>, an Armsmaster with a billion power, it will still be what the cap says.

There is a variety of other information about the Foresight-related glitches in that thread - notably, don't equip a glitched one as your Jump will be reduced to -100 and you can't move.

Story Characters are great to migrate the first 5 times because you get a 5% aptitude bonus for all stats the first 5 times; aptitude % means you gain that much of your equipment's stats as a %. So after 5 transmigrations you get 25% more out of all equipment for that character!

However, transmigrating shouldn't be done lightly unless you can very easily level your characters. You'll have to level your characters over again and gain enough mana for them to transmigrate again.

The main reason for transmigrating to Genius is because of the Inheritance Rate. Inheritance rate is how much of your Skill and Weapon experience you lose when you transmigrate. If you have 1000 Ax Skill and transmigrate with 50% inheritance rate, you now have 500 Ax Skill. Since there is no easy way to grind Weapon Level or Skill Level, losing skill level really hurts. Doing it early on can be good if you're sure you can get the mana/levels back in reasonable time.

If you can't easily get enough mana to migrate your character as a Genius, don't transmigrate yet. Disgaea is not about grinding, it's about grinding intelligently. You're too early in the game to worry about this from what I can tell.

Ability points don't stack in the way you describe; you always get 10 for genius, and you get a set amount of points extra for that character based on their total level.