if you intend connecting them in parallel then they must be matched i.e. same type from the same manufacturer.
The centre bolt must not create a shorted turn. Insulate it carefully to ensure the top and bottom of the bolt can NEVER touch the metal case at the same time.
The rubber rings are there for vibration.
The "shorted turn" comes into play when you create a circuit that wraps around the toroid much like the wires inside it do.
Shorted Turn Condition
A completed path by any conductor passing through the center of toroidal transformer, around the outside is a shorted turn (eg: the top of the mounting bolt shorted to the chassis). As with any short circuit, this condition will give rise to high circulating currents, and high heat. The transformer may be damaged beyond repair.
You are allowed to connect ONE end of the mounting bolt to the chassis. The dish usually formed in the big washer is there to keep the bolt head from projecting above the transformer insulation. This helps by preventing the case from touching the free end of the mounting bolt however the washer is still exposed to contact. The case or other chassis metalwork must NOT touch the washer or bolt head.
You could add a layer of insulation over the top of the washerplate and bolt head to prevent inadvertant shorting of bolt to chassis particularly while dismantling or assembly with power on (dangerous pastime).
A shorted turn is a complete electrical circuit from the bolt bottom to the chassis to the side panels across the top (lid) down through the bolt head back to the bolt bottom. If this circuit is completed then the turns ratio of the transformer becomes about 200 to 1 and although only a small voltage is generated, maybe 1/2 to 1 volt the resistance is very low possibly 1 to 10 milliohms the current could run to KA in parallel to the true secondary winding and will definitely overheat your transformer.