Q1: Is it OK to support pipe from the pipe flange (the TOS of the pipe support is exactly under the bottom of the flange).
Q2: if the answer of 1 is yes, is it advisable to use the flange bolts to attache pipe shoe to the flange in order to support the pipe.

I have seen flange supports. They are pretty common of FRP pipe especially. And if you do support from the flange, it is generally a good idea to make a half circle plate that bolts to the flange and ends in a pipe shoe.
I added a page from the Maverick Applied Sciences pipe support catalog to show you what I am talking about. They are generally for FRP systems, but I have seen such things fabricated for steel pipes too.

Thanks,
Duncan

Yes, Flange supports do exist in Steel pipes as well. It can either be a cradle type, or a gussets type. Cradle type provides considerable greater support than that of gussets type. Main advantage of this type of support is we placed them exactly where the load is acting (i.e. at valve/flange location). It does reduces vibration as well.

To your second question, yes, it is allowed to use flange bolts. Since the plate will have a hole which should match with the flange holes and BCD of flange.

Regards,
R.K.

For heavy and high rating valves such as trunnion mounted ball valve will be required supports at flanges not only FRP application.