Assuming a carbon fiber fuselage, I'd put the transponder and PowerFLARM A antennae in the fin. The COM antenna is in the rear of the fin. There should be one foot separation among the antennae.

You will need something like LMR-300 cable to the Flarm, and perhaps transponder, antenna in the fin to limit loss to acceptable values. Get a Flarm Antenna with a short lead.

Consider an access port in the fin.

It would be really nice if glider manufacturers included a conduit to accommodate future cables between the instrument panel and the fin.

The PowerFLARM ADS-B antenna will do just fine ahead of the instrument panel. Transponder signals are considerably stronger than Flarm signals.

It won't hurt to have a Flarm B antenna ahead of the instrument panel. The Core 1.1 can transmit on both antennae if you purchase the RFB option.

If installing ADS-B Out, you may be facing extra requirements for the GPS antenna. Plus the vario and Flarm have their own GPS antennae. A splitter may help.