Can we get real time prices for FX in milliseconds using EIKON API in c#?

Eikon gets streaming market data from Elektron, so the data is essentially the same whether you get a datafeed or a market data terminal. The most important difference is with Eikon being an end user product you're licensed to use any data you retrieve from Eikon for the individual user's use only. With Elektron datafeed you can use the data in enterprise or server side applications. There are numerous other considerations that may or may not be important for you: market data infra, conflation & latency, technology stack, application performance (however you define it) etc. The best way to go about selecting the right product for your use case is to speak with your Refinitiv account manager as @Zhenya Kovalyov suggested. If you're not yet a Refinitiv client and do not have an account manager, you can either call Refinitiv office in your country or use generic sales contact form following the link below.
https://www.refinitiv.com/en/contact-sales

you will need to publish your additional question as a separate post on the elektron forum.