tiny creature riding a small mount

[vhok]

lets assume its a small humanoid, 2 legs 2 arms etc. riding a small size cat using a starknife. am i able to attack into adj squares with my tiny humanoid without a reach weapon? if not is there any way to do it without changing my size or weapon.

[avr]

Your space changes when riding, your reach from that space doesn't. No.

You can get the lunge feat or the long arm spell, and there are ways to make entering an enemies' space easier.

[Nikkok]

If you want to be able to attack your opponents without reach, Escape route feat, shared with your mount, will help you to avoid AoOs while moving into the opponent's square.
Either play Hunter, or get the Horsemaster's saddle.