Try to answer some of your questions:
1) is it possible to create a VCP based LAG of 2*10G links between 2 members of a unique VC with 2 different link types :
a) 1 link based on SFP+ 10G optical ports of the expansion module
b) and 1 link based on RJ45 10G copper built-in ports of the 32ports EX4550 base module
or do both links inside the VCP based LAG have to be of the same type ?
on the drawing below, see for example the "vertical" VCP based LAG between member 00 and member 01 on the left side
=> Basic answer is yes. Members of a LAG do not have to be the same physical link type. This is a LAG/AE principle and is independant of whether used for Ethernet or VCP/Ethernet. I believe they actually could be of different speed and still work. Except maybe if you enabled 803.3ad/LACP on the LAG/AE as I believe this standard requires all members to be of the same speed, but I am not 100% sure of that. Check the standard could answer this for you, but at same time there are very few instances where most people would use mixed speeds.
I do know that in VCF, mixed speeds with the uplinks are actually supported, although not recommended, but VCF has a more advanced link load balancing algorithm than VC.
2) is it possible to have a VCP based LAG between 2 pairs of EX4550 in the same VC ?
see drawing below for the "horizontal" LAG (based on 2 SFP+ 10G Fiber links between the 2 DCs) that joints the members 00 & 01 pair on the left to the members 02 & 03 pair on the right ?
Of course. An Ethernet based VCP link is allowed within any One complete VC. This is one of the major advantages of Juniper VC vs other stacking technologies. This allows for geographically disbursed members within the VC. A VC could actually be created over a Ethernet WAN link as long at the round-trip delay is less than 100 msec.
No matter how you form the VC and what type of VCP connections you use, built-in or Ethernet based, the underlying IS-IS protocol knows how to build the topology and determine best [shortest] path between any 2 members.
Hopefully this provides the answers you seek.