Is it possible to use this with a dildo?
No, the balls will help hold it in place, even if it's vac-u-lock compatible.
I understand that the Plug ca will be removed, but I don't know which size of ring they include.
Two rings are included, one is a 1.5" inner diameter and the other is a 2.
Is it possible for this to be used by a man?
The harness has a high opening under the plug that allows access to the genital and anal areas while it is worn by men or women.
What size dildo can the orings support?
If you are using a flared base dildo, there are 3 rings that will fit most girth sizes.
Is it possible to use any dildo with this, or only the vac U lock hung?
Yes. Any dildo with a flared base can be used with this as you can switch the plug out and place an o-ring on instead.