If you set http client proxy, all http requests will use that proxy.
If you use custom http client module, I guess you need to set up
proxy over that custom (extended) http client module.
You can conduct a simple test and determine how those 2 modules
interact, or if they ignore each other.
Maybe you should ask your question to author of custom module,
under respective custom module thread, if you are too lazyto test
it yourself.