Gst on reverse charge

Hi,

1) I suppose to pay GST for payment / bills received from unregistered dealers. Is it necessary to pay GST under RCM for the bills received from registered dealers,but not received GST bills, since they didn't crossed threshold limit.

2) Once i paid GST under RCM for the month, when i can utilize that credit... Ex. If i pay in June 19, can i take such ITC on same month of subsequent month?

Please clarify