So they can use combat pool and special attacks but what about regular ones, do they just default back to the attribute there by raising their TN?

No, they attack using their reaction and do not increase their TN.

Dont forget they have different reactions in astral and physical just to make it harder.

For simplicity you may wish to just rule the higher force spirit wins in astral, with great form beating normal spirit dispite force. With a big chunk of ref largess on interesting match ups.

Somewhere at the start of the critter chapter in the BBB (IIRC, it's been a while) there's a rule that critters never need to default, but just use their reaction for combat tests, if they don't have a specific combat skill. Spirits are critters, so they get to do the same.

Awesome, thanks! I knew I was blind!