Hi,
Yes. One can easily utilize ECS for RFX & Auctions.
But in classic Scenario things are not as straight forward as in ECS. When one uses classic scenario it is very well understood that Contracts, RFx done at ECC.
I am not denying; that classic scenario is not capable of doing contracts or RFx at SRM end. It can very well do. (I have created contracts at SRM end.) But one can't transfer it to backend. So in a way those contracts are local contracts.
I will suggest; if you want to use RFx, Auctions; prefer ECS. It will reduce complexities in future.
Regards