attachments is network diagram .
Cisco 6509 run PVST+,
Cisco nexus 5010 run RSTP .
S5 is root bridge .
stp loops will occur ?
PVST+ and RSTP will work fine together but you need to be aware that any vlan that exists on both the 6500 and Nexus switches will revert to standard 802.1d timers ie. you lose the benefit of RSTP.
Apart from that you should be fine. One thing though, the 6500 supports RSTP, is there any reason why you cannot use RSTP on all the switches ?