I have searched for info on whether or not this is a common practice or a risky practice, your thoughts?

What are you using for a underlayment ?

Very risky if the UL is particle board.

Underlayment is particle brd

Remove the particle board. The 3/4" plywood should be fine for a subfloor as long as the floor joists are 16" OC.

Gary,

What do you mean 3/4" plywood is fine if the joists are 16" OC ? What does the 16"OC mean ?

I have 3/4"plywood, with joists about 15" betwen them. Is this the same as 16" OC ? I'm installing 3/4" by 21/4 strip oak.

The plywood floor squeaks alot. It has old rusted nails. Should I use 2.5" screws only or should I use 2" ring shank underlayyment nails ? or a combo of each ? I want to make sure teh subfloor doesn't squeak or pop anymore!

thanks.

16" OC = sixteen inches on center. yours are that. Screw down the plywood with 2" screws and a cordless drill/driver. That should take most of the noise out of the plywood subfloor. The new wood flooring must run at right angles to the direction of the floor joists. Subfloor should be flat as well. 3/16" in a 10' span.

After you have re-screwed the subfloor, you will have to go around with a hammer and re-set all the old nails, they will be sticking up.
If they are rusty, as you indicate; is probably historical damage. perhaps at one time carpet was laid over this plywood and shampooed too often or with too much water. I dont think it is of concern, unless the surface veneers of the plywood are de-laminating. If that is the case, consider adding a 1/4 inch plywood underlayment over the space before installing the hardwood. This can be simply stapled down to the existing sub-floor, after refastening.